Those little pet peeves that, when your opponent does them, make you want to send a Leman Russ through their skull.

Firstly...

Flicking... A tendancy of some people,usually children, to flick your models when they kill them. Plastic ones, hell, I do it myself to my own, but a metal miniature can chip, and my metals tend to be better painted than plastic.

Nose picking/sneezing on play area. It always congregates on your tanks, and you never notice until you pick it up. Nurgle players are especially bad for it

Borrowed models not being given back. Loaned a guy a space marine or two, from a display squad I'd painted up. Really classy job, even if they were just plastic marines. Nice Missile Launcher dude and one bolter guy were missing from the case when I got home.

Obscene Beardyism: That Tau player on the one inch ledge being a case in point. Above model height you have an argument for a difficult terrain test... but otherwise...

And worst of all...

People who kill your tanks on their side of the tab;e, and refuse to hand them to you. Some are genuinely afraid of damaging them, fair enough, but if I ask for a dead model to be handed to me, it seems no big deal to actually do it... Saves me reaching across the table, or saves time from walking right around the play area. In a crowded GW when you're 18 stone, this can be an issue.