There's a thread on SPUF about the placebo effect in TF2. I decided that this topic is interesting, so I'd like to share some of my experiences with you. You can join in and add some of your own experiences in the comments.

My experiences:

General:

Random Crits are cheap: 70% of the times I died, even if I would have died anyway, it's to a random crit.

Sentry Annoyance: Whenever I tell my team that there's a sentry ahead, they run in and get themselves killed.

Scout:

FaN vs Soda Popper: Soda Popper just doesn't really seem to do more damage when hype is active than the FaN. Maybe it's because I'm moving a bit too much to get decent meatshots with the Popper...

Shortstop trouble: Just doesn't feel right.

Lugermorph vs Pistol: Reload on the Luger seems quicker, but the pistol seems more accurate.

Fish-o-doom: Seems to have a slightly higher rate of "fire" then the stock bat.

Soldier:

In-Direct Hit: I can't seem to aim it properly at moving targets.

Black Box Bashing: I don't feel like the 15 health heal actually makes any difference.

Shotty vs Reserve Shooter: I seem to get better accuracy with the Reserve Shooter.

Banners Bad: I seem to die a lot with these.

Melee Weapon Woes: I don't seem to hit with Melee weapons as much as a soldier as I do with other classes.

Pyro:

Spy Panic: I get a bit more annoyed at teams that don't spycheck when I'm a pyro than when I'm any other class. I guess it's because I watched Meet the Spychecker too many times...

Calling Haax for nothing: It seems that enemy pyros somehow get a damage bonus against me - even if I take the initiative, I'm the one who dies.

Flamethrower of Choice: If I'm not using the degreaser, I can't seem to do anything.

Shotguns vs Flareguns: Shotguns seem to keep enemy pyros away.

Shotgun vs Reserve Shooter: See Soldier.

Flaregun vs Detonator: When using the flaregun, I get headshotted every time I take a shot, no matter where I am. This has got to be the weirdest thing on the list.

Axtinguisher vs Postal Pummeler: When fighting against pyros, it seems that the postal pummeler gets better range than the axtinguisher.

Rockin' the Sharpened Volcano Fragment: I tend to taunt more with this weapon than with other melee weapons. Maybe it's because it just looks awesome?

Demoman:

Demoknight vs Demoman: When I play Demoman, it seems as though enemy Demoknights actually target me. When I play Demoknight, it seems as though enemy Demomen target me. Anyone else notice this trend or is it just me?

Caber - The Anti-spy: I get more spy kills than normal with the Ulipool Caber.

Half Zatochi on a Demoknight: Barring DeGroot Keep, I can't seem to kill anything with this!

Heavy:

Dah, this won't work: I could play him back when I first started, but not anymore.

Minigun vs Iron Kurtin: The Poker Night achievement weapon just seems more powerful for some reason.

Engineer:

Stop picking on me!: The longer I play on one session, the more people seem to be going after me. I do tend to play a good engie, maybe I'm causing rage?

Widowmaker vs Shotgun: I rarely if ever run out of metal with the Widowmaker, but I'm always running out of ammo with the regular shotty.

Fronteer Justice vs Shotgun and Widowmaker: I generally do better with the shotty or Widowmaker equipped, but I get more shotgun kills with the FJ.

Lugermorph vs Pistol: See Scout

Southern Hospitality is OP: Pyros never attack me for some reason, even though I take MORE damage from afterburn with this thing...

Regular sentry vs Mini-sentry: I tend to get more kills as a Gunslinger engineer, and in no way is the mini-sentry NOT a factor.

Medic:

Follow the Leader: When I join a server, our team usually has no medics. By the time I leave, we have at least two medics on the server besides me. Coincidence? I think not!

Battle Medic Blitz: In the above situation, one person always tries to pull a Rambo as a medic at least once. They die horrifically.

Kritzfail: I can never get a Kritzkrieg uber off.

Sniper:

Sydney Sleeper Ownage: I seem to do better with this Australian tranquilizer gun than any other rifle.

Huntsman vs Rifle: I can't aim for nothing with any rifle that isn't the sleeper, but I can nab decent shots easy with the huntsman... without spamming!

Spy:

Spy-cicle is False Advertising: I die less in general with the Spy-cicle than with any other knife, however when I do, it's almost always to a pyro. Also, the spycicle doesn't help me against pyros. They chase me while I'm disguised even though no one else does during the flame immunity period.

Knife Konundrum: I can't seem to get any unlockable knives through random drops.

Revolver vs Anything Else: I get kills with every revolver except the stock gun. I can't kill anything with it.

Sapper Panic: nine times out of ten when I sap an engineer's equipment, they try to save the buildings first before defending themselves. Doesn't apply to bots or when the enemy knows I'm there.

Sapper Deaths: Half the time when I try to sap an engineer's building, a pyro burns me before I can even place the sapper.

Medics are idiots: When I kill an enemy while being healed by an enemy medic, I somehow get away with it. This can happen multiple times during the same life, and sometimes I even get ubered during the whole deal.

Medics are Geniuses: When I try to backstab an enemy medic that's healing me, I get killed by that medic even if they were blindly healing me while I was killing their teammates.

Random Drops:

The Cake is a Lie: I never get what I want.

Surprise attack coming from a box: Whenever I get ambushed and an item drops, it's ALWAYS a crate.

Key Fraud Scandal: Whenever I get rid of my crate(s) by unboxing them, a crate is the first thing that drops and it usually happens during the same session.

Mann Co. Fraud Scandal: Whenever I buy something from the Mann Co. Store, I get a second one in my next drop.

Steam Friends:

Mad Hatter: I seem to attract hat traders for some reason when I play as a engineer.

Clan Stuff:

It's =4ciD/=, not .4cid!: Some people think I'm .4ciD the CoD master due to my clan tag, but only on clan based pubs, not anywhere else. I usually prove them wrong.

Other experiences: