Required knowledge on playing ______

[Echoen]

This thread devoted to stopping common and/or terrible mistakes when playing any class. Add your own tips, too!

Engineer
Don't build your sentry close to another engineer's sentry! Keep them apart from each other. It's so aggravating to see two or even three sentries all snuggled up to each other. Not only does this mean that anyone can kill two sentries for the price of one, but it means that two sentries are guarding the same point when they could each be covering different areas.
Please don't forget to build your teleporter exit, it's very helpful in offense and defense!
Why sit behind your sentry the whole map and bang on it endlessly even if you're not being attacked? You have a shotgun, go out and help your team! Your sentry allows you to defend two places at once. Even if you're just a measly 125 hp with a shotgun, you really can help out - as a distraction to the enemy, if nothing more. Plus you'll get training on being effective with the shotgun, and trust me... learning to use the shotgun well /really/ is a big kudos! And so useful.
If you see a demoman laying stickies under your sentry... get the fuck away! Don't just sit there and bang on your sentry, it only means you'll get blown up too. Stay alive, and rebuild the second after your shit's been kabewmed. You're a lot more useful alive than dead.

Medic
Don't stand still, even if you're ubercharged. Run around! Draw fire! Then duck behind your allies when the uber goes away. Also, keeping on the move makes it that much harder for spies and snipers to kill you.
When caught in a firefight.... HEAL! Please do not switch to your saw or needles and join the fray. Too often do I see medics get "Rambo-itis" and rush in to get some kills, and leave the rest of us to die. Even worse when I'm depending on that healing to keep me alive while I take the enemy down - aka, do my job - and it's like the medic just yelled "Chinese fire drill"
Use the Ubersaw. There is just no good reason not to. The only thing the Bonesaw has for it is a faster swing rate - paltry when compared to gaining +25% to your ubercharge per hit! It turns that spy into a free uber, or allows you to get in a whack and then escape with your life.
Be aware. Don't get tunnel vision. Pay attention to teammates who call for Medic. If you hear someone call for medic, pay attention. Once you get that down.... then you can start checking for spies. How? When spies cloaked as your teammates call for medic, the animation goes off, you get the sound, and you even get the speech bubble of the red + sign.... but in the chat window, the "So and so yells, "Medic!"" Does NOT activate! I've caught countless spies spamming for medic in the act.

Spy
Learn how to do fake-outs when cloaking. If yuo're fighting an enemy and it's going badly for you and you want to get away, it isn't enough to just cloak and flee... it's very easy for everyone to know where you're going if you cloak and run the same direction you were going before you cloaked. Learn to run a little ways then instantly backtrack, and practically hold your breath and squeeze against a tiny little corner or spot that your enemy has already gone past and checked. It is SO funny to watch enemies unload clips into where they think you were or thought you were going, when really you never left them at all. They either go off chasing you or stay paranoid for a while longer... an excellent opportunity to go "SURPRISE! I never left!" and backstab them.
Don't be afraid to use your revolver. That thing is fairly accurate and does some nifty damage. The damage has been nerfed recently but a good few shots will bring down your foe decently. Especially if you crit.
Be aware of the enemy team's composition. If they have no pyros, or just one... don't cloak as pyro, even if it is easier! Cloak as spy, sniper, medic, pyro, demoman, engineer... and yes, there ARE times when disguising as a heavy, soldier, or scout are appropriate!

Sniper
Remember lagging hitboxes.

Pyro
When in a fight with someone at point-blank range, and both of you are running in same direction... remember, even if your fire is ON their body, it's not hitting their hitboxes! On their screen, you're firing right behind them. You need to lead your target a little bit... once I learned to do that my kill:death ratio as pyro went up fast.
Weigh the cost/benefits of Airblast vs Backburner. Backburner - murderlicious when you come behind someone. Airblast - reflect rockets and grenades, push stickies away, put opponants off ledges, turn enemy ubers into uselessness, bounce enemies in the air, dislodge an engineer from his sentry, disorient an enemy... the list goes on. It's my personal opinion that the usefulness of Airblast completely outshines Backburner - too often do I see situations in which a pyro is FUCKED because he doesn't have the option of Airblasting stuff. But it's up to personal preference - if you're one of those W+M1 pyro players, backburner is for you since you don't have the intelligence (or care) to think in the first place.

Heavy
The couch is MINE! YEEEEEAAAAAAAAAAGH!

Soldier
Aim at their feet, not at them. Aiming at the target other than a sentry means they will just dodge it. Also remember to lead your target.

Demoman
Remember to laugh. Crazy shit happens when you're playing a demoman. Both to you, and from you to others. Keep it cool and Demoman turns into a barrel of laughs.
You can 'charge' your stickies to shoot them farther. Just hold down the fire button, and you'll see a small white bar under your gun going up. You can shoot a sticky a fuckton far away!

Scout
Double jumping can change your direction
When you see an enemy, immediately shooting at them isn't a good idea! Your shotgun does piddling damage at medium/long range.... but a FUCKTON at close range! Get behind the baddy and run up really close and fire off a few rounds, then run if you don't kill them.

[Angry_Squirrel]

A big one to add to Pyro:
FLANK
Yes, even if you have a backburner, learn to flank! It makes you MUCH harder to hit as a target, because let's face it not everyone dies to a few crits from behind and requires further cooking. Flanking is even more important in pyro vs. pyro, as you don't get the advantage of additional burning damage to the fire damage. Of course, as Ailure already said, leading the flames is a big part of hitting your target while flanking, so don't forget to do that.

[Ayeaka]

More for medics.

Firstly FOR THE LOVE OF GOD, if two people running toward you are on fire, and close to death, and one's a medic...
SAVE THE MEDIC.

Especially since THEY can heal the other person, while you focus on making sure they don't have to choke out a possible 15 second respawn time before they can get back to healing people. Even if they -can't- get to the second injured person in time, that's a good chunk of time less that it'll take for said medic to get back to the front lines, doing their job.

ALSO.

If you're running up to heal a group of people that have just been ambushed (very common when a group is attacked by pyros,) you DO have a gun, and you CAN help them before you're in healing range.
Too often I've seen people keel over before a medic can get to them, when it could have been prevented had the source of damage been eliminated only a split second quicker. Q is your friend. It's easy to fire until you're in healing range, then rapidly switch and help your teammates out that way. DO consider.
Ten damage per hit, ten hits per second.
100 damage per second can do quite a bit of good when you're running in.

[Angry_Squirrel]

[Dravu]

Also for snipers:
1. Strafe! Still targets are the easiest things for other snipers
2. Don't be afraid to unscope and wait for someone to come into view. Being fully charged doesn't help if you don't even see the person running around.
3. If going up against another sniper, try to keep them in the middle of your screen. Other snipers can be quick-shot, so keeping them near the middle of your screen will allow you to quickly zoom in, target their head, and shoot.
4. Scouts are annoying as fuck. If you see one, don't be afraid to bodyshot.
5. If you're sniping near another sniper, try not to run around in front of them. I personally try to stay behind other snipers if they were there first.
6. Don't be afraid to pull out your Kukri and kill some people. It's a pretty nasty weapon at times and is good for close-combat. Don't pull out your gun for close-range unless it's something like a pyro where flames kind of hurt.

[EuroVen]

To add more on the engi part from Echoen, I found it to be useful, if you are trying to outrun an enemy who you think is going to chase you and if you have metal to spare, as in 100 or 200. Build your dispensers behind you and take your run, chances are they are going to attack the dispenser than you running away so this will give you a good amount of time to get back to your team for help, only if the character isn't a Scout. Though they have to be a hefty far away if it is a pyro, soldier and demoman people, since they will massacre it in seconds. As another part instead of the shotgun, use the pistol, obvious reasons being you have a shit load of ammo, 200+12 compared to the Scout's 32+12. In one of those clips you will garner crits, and I must recall pistol crits are insane; side note, if you see that you've gotten one crit bullet from your remanding clip say 8, do not reload as your crits will disappear, I saw is that once you get a crit bullet it goes on for the remainder of the clip or for a few seconds.

Scout, I don't have much to say on this besides, jump, crouch while on the ground, squirm around the enemy you have the speed to get up and running. If you are in a gun fight with another scout, jump and crouch while on the ground frequently, his shots are easily evaded from what I've noticed when I've gotten in the Scout mood. Also, don't underestimate the power of the pistol for it will not fail you at times; especially for those awesome bullets. It can and will put an enemy to shame.

Pyro, if you want a nifty little trick to do when going into a crowd if you like to be in the fray. Do a 360 while firing away, or do more if you want to have fun. Instead of just catching probably 2 on fire, why not grab them all? If those are who are still having trouble with the airblast and can't seem to get the right timing, you might want to do it within a good 5-10ft radius.

[DrJones]

Soldier:

Also remember to have a height advantage over your target as much as possible. It allows you for a better angle for your rockets, and makes things easier.
Remember to switch to your shotgun if you're caught without rockets.

If you're attacking and run out of rockets, consider backing off a little to fool the enemy you gave up, only so you can reload more rockets. The soldier needs approximately 3.8 seconds to reload all four rockets.

[Taross]

Heavy:

Doctor may be friend, but even non-medict-backed heavies can have a devastating effect. Specially when time is critical, don't stand around in the spawn waiting for a possible medic to buddy up with. Besides, in most capture points maps without engie teleporters, any medic will catch up with your fat ass anyway.
When you're on the move take out the shotgun (or the melee if you have sandvich). It's faster response when you run into an enemy than having to spin up your minigun. It takes about a second to start shooting with Sasha, a second in which you're dying.
When you get close to the fray, spin up using your secondary button. This way you can start shooting as soon as you round a corner.
If you crouch and use the gun, even just spinning empty, you can NOT move.
This one's useful for other classes too, but learn to keep half an eye on the kill-messages in the top right corner. If you see the icon of the switchblade knife flash by, you might still be lucky enough to turn around in time to prevent the spy from backstabbing you.
Listen to the team, listen to your medics' cries for help.
Learn the map shortcuts and odd ledges... The first may be obvious, but the second one is priceless. My signature doesn't say "SUDDENLY! HEAVY!" for a reason. If you learn the odd ledges and locations where you can get the drop on people and use them, the other team will soon curse your name as that damn fatty that just won't die and appears around every corner or even falling from the ceiling, shooting at everything in sight.
As for range, pretend that your minigun is like a shotgun without the reload. If the enemy is too far away you're better off throwing spitballs at them instead of bullets.

And the heavy is one of the few classes who can sneak in a kill at round lost or failmate. After all, if you're on the move you should have your fists or shotgun out anyway. Taunt them to death.

Medic:

PLEASE PLEASE PLEASE for the LOVE of all that is SHINY... You're at the front, your uber-bar is full, your target (heavy) is saying "Ok, doctor, hit it!" through the mic and runs out in the open."... DO NOT BLOODY RUN AWAY! So many times have I charged off into the fray with an uber-ready medic on my heels, only to turn around and notice he's all the way in the back again healing a scout.

And since we're left on Scouts...
Teleporters:
Spies and Scouts please do not use a teleporter. Spy, you will be visible thanks to the sparkly christmas lights at your feet. Scout, you're fast enough to run. Don't steal the teleport off of a heavy. And yes, even with the upgraded teleports that's still annoying.

[EuroVen]

[Purplecat]

They changed that over 9 months ago! On the big medic update! (And yes, I still get a lot of "Spies shouldn't use teleporters!" cries towards me when I'm playing Spy. ._.)

And scouts can use teleporters fine if it's a level 3 one. It recharges so fast by then it's not really a big deal unless everyone respawned at the same time.