I get annoyed by the classic There/their/they're and your/you're mistakes, as well as than/then and such sort of things. However, it is reduced to nothing by my hate for People Who Write Everything Starting With A Capital Letter. Or pEOplE ThAT aRe EvEn WOrsE tHan THAt bY raNdOmlY CaPITAliZinG AnY LeTteR TheY wOUlD liKe. This last sentence is written assuming they have any sense of grammar. Of course they do not, since it's always something like "pPl DaT R EvUn WsRE tHen dAT cOz raNdOmlY KaPtaLZiN AnY1 LeTtR TheYD liEk".
Also, though this is not about the grammar, I get mental breakdowns and suicidal urges from people stating the obviously unnecessary dumbshit retard comments like
"Everybody I Have Something To Say Im Just The Best At This Game So Leave Out Now Lol I Mean It"
and someone replying with
"Everybody don't listen to *censored username*,your not that good dude,get over yourself man!!"