Ha ha, that reminds me. In the Math department at my college, a sign posted read "Did you loose you're bracelet? Come buy room 320 and identify if you want it." I wanted to rip the poster down.
My main pet peeves for grammar are the you're/your, too/to, their/there/they're, and when people use "shorthand" Internet speak. I hate LOL, LMAO, IDK (a new one, means I don't know), and other various Internet crap. I also hate it when people say tho/kno and atm. In other words, I hate Internet abbreviations. I understand when people use them because it saves typing time, but when people that type fast use them, it drives me insane.
Two new atrocities are meh and feh. I fucking HATE those two words. I can't stand them at all. My friend at college said them on AIM until I told him to stop in real life. Something about those two "feelings" or "state of beings" annoy the shit out of me.
I hate ur. It's not even abbreviated; this is one of those words that are completely made up based on how the original word sounds. When people say "ur" to me on AIM, I ask what the hell a "ur" is. Is it that hard to type you're? Or even your, if they fuck up worse than usual?
And for the finale: plz thx. Holy shit. If I see anyone saying this that I talk to on AIM, I go apeshit on them. I ask them how it's faster to type the z and x than please and thanks. PLZ doesn't even resemble please. PLZ would be pronounced something like "pillz." And the God-forsaken THX. THX is a theatrical sound encoding medium, NOT an abbreviation for thanks. Again, THX would be pronounced as "Thux."
Maybe I went a little overboard, but perhaps this thread really needed to be made for people to vent. There's my ventings.

Oh, and I don't mean any of this personally, I'm just stating what irks me. Have a nice day!
-Lasereth