Right when I'm about to backstab, the target rotates and it winds up just being a facestab that costs me my disguise and pisses off whoever I stabbed, usually a heavy who mercilessly tears me down with his machine gun. Of course this magical skill evaporates when it's me getting backstabbed.
Getting a perfect backstab ruined by a Fail-A-Nature Scout's last half-second spycheck. Your cover is blown, so is half your health, then the medic ubersaws you and fills up his ubercharge.
Bullshit levels of damage from something that isn't either a backstab or a crit rocket/sticky. As a spy, a sniper once killed me in one blow with a hit from his kukri AND I HAD FULL HEALTH.
Situations like "You check to the left, the right, and then behind you, nobody's there, you proceed ahead, confident that nobody will be there to spycheck you and blow your cover, then suddenly, out of nowhere, a huntsman arrow clips you in the leg. Half your health is gone, you retreat from whence you came, only to be hit by a flare on your way out, now, burning and almost dead, you barely make it back to a safe place alive. And what's this, a medic is coming, you're saved! Oh wait, you spycheck him and an enemy spy falls dead where your salvation once was, you continue retreating, but then you get backstabbed, it was just the dead ringer trick."
That the crit rocket/sticky/Huntsman arrow has 999% more accuracy when I'm just about to reach a health kit.
Demomen who boast about their skill when all they do is sit at some critical pass and camp with stickies.
That enemy spies have the magic ability to tell when you're too far away to knock off a sapper, as opposed to say, just getting metal from a nearby, but out-of-sight dispenser.
Engies who always build the same stuff in the same place and get mad when a demoman takes it down in record time.
Fellow engies who refuse to help you with your stuff, indeed, you just work on your dispenser while I'm trying to run all the way back to my sentry that you're right next to and it has one bit of health left.
Getting headshotted while cloaked, if it isn't an aimbot, then it's just a bullshit annoying coincidence.
People who spycheck with their most dangerous stuff, and it seems to only be when they're spychecking you, after they blow you up with that crit rocket, they'll just switch to the shotgun and take a few shots at people.
Ultra-long respawn times, and the 20-second ones only seem to be on the maps where you have to walk a billion steps just to go anywhere.
The pyro who seems to know right when you're about to backstab a medic with full ubercharge and ONLY THEN decides to give you a puff of flame, and it's always the pyro with the Backburner equipped (for those of you not in the know, the Backburner gives 100% crits if you hit from behind).
That one crappy medic who will heal everyone but the most important people, i.e. a burning medic who's about to die.
Getting set on fire really close to spawn and dying two steps away from the health/ammo refreshment cabinet.
That you ALWAYS get a bout of crits when you don't need it, I cannot tell you how many times I've been in the thick of it as a heavy and not seen any crits but been shooting at some wayward scout and gotten crits. This of course does not apply to the soldier you're facing off against who's about to fire that crit rocket at point blank.
Hiding in the spawn zone as a medic with 100% uber, waiting for your buddy, then having someone enter with a demoman in hot pursuit who one-shots you with a grenade.
Getting killed in any way at 100% ubercharge, though so far, I've determined that the most annoying way is sniper headshot, it's not like a backstab where it's your own fault that you didn't check the guys behind you, but often it's just a case of letting your guard down for one split second before that huntsman arrow guided by the god of aiming himself pops you in the head right before you were going to attack that sentry with your medic buddy.