What really torques me the most though is when Engineers deliberately put their turrets in obscure places (for example, 2Fort - in the water, by a corner of the center pond so that it shoots up to the top of the bridge and the side nearest it without revealing itself, and onto the front deck of the base opposite to it) so that it'll only get kills when a very specific fight breaks out nearby, and ends up snatching every kill because of it just so they can power up their Frontier Justice... and then realizes that because of where the turret is, no one will blow it up and he'll never get his crits.
The good news about this though is they'll usually put it back in the same spot, so when you DO find it and break it, just hang around a bit for him to come back and start rebuilding it and murder him dead. No crits when you're dead, jackass.
I get bitter about battle Engineers for some reason. Probably because of how goddamn cheap the turrets are. If you have a tiny spot of lag, you're fucked compared to a turret. The only way I've been able to destroy turrets is either the tried-and-true Demospam, a Soldier from long range, or god forbid I come around the corner face to face with one as a Pyro and react quick enough to stay in front of its circle-strafe. And on top of that, they have just ridiculous amounts of bullet-shove, even at level 1, or even as Mini-Turrets. They're already a formidable obstacle as a defensive measure - when used offensively, it's like a giant 'Fuck you!' to the enemy team. If it's a level 3 sentry, they have to focus it down - and meanwhile the Engy runs away and starts rebuilding. If he's a Wrangler Engy, the turret is so damn durable and that shield stays up for three seconds after switching to his wrench to reload and repair it, and since it only takes two wrench swings to repair pretty much every Engineer building ever, that's plenty of time to switch back to the Wrangler and let 'er rip. Of course, Wrangler Engies tend to have tunnel vision and are susceptible to Snipers or Spies, so that's an easy counter - especially for a Spy, since lololol sappers when the shield drops. But otherwise that damn turret is in my opinion the single strongest weapon in the game, whatwith its perfect accuracy and nearly flawless reaction time.
Damn turrets