What grammatical errors really piss you off?

[prodonkey]

Loose and lose

[HoneyBadger]

[ QUOTE ]
Loose and lose

[/ QUOTE ]
Guilty. I fixed that leak now though. (Anyway, I'm not a native speaker, so it's not as bad...)

[viper930]

Excessive/incorrect use of punctuation, especially commas.

Perfect example taken from good2cu's website:

"DonButtons, is a balla for real and recently purchased a Lamborghini, at 20."


Also, people who use your instead of you're and mix up their with there.