Quite a few posts specifically referred to language/writing.
It's easier to do pet peeves in categories, so I'll stick to languate-related. I have more of those than almost anything else.
To reiterate or expand on some other posts:
your/you're
it's/its
their/there/they're
Non-existent words like their's/her's
Some non-pronoun gripes:
A. than/then
B. Hypercorrection, such as incorrect use of "I" as an object ("They rode with my mom and I"). Sometimes, the same person would also say, "Me and Betty went to the store."
C:
PEOPLE WHO WRITE EXTENSIVELY AND EXCLUSIVELY IN ALL-CAPS WHICH IS AKIN TO SHOUTING IN TEXT USUALLY FOR NO GOOD REASON AND I'M NOT JUST TALKING ABOUT MESSAGE BOARDS BECAUSE I'VE SEEN IT OTHER PLACES TOO LIKE LETTERS TO THE EDITOR AT NEWSPAPERS WHERE I HAVE WORKED IT'S AS IF THE CAPS BUTTON ON THE PERSON'S KEYBOARDS HAS MALFUNCTIONED AND SOMETIMES YOU GET A BONUS WHEN IT'S ALL WRITTEN AS ONE SENTENCE AS IF PERIODS DON'T EXIST AND THE WRITER WON'T EVEN USE AN OCCASIONAL COMMA AND WHEN IT FINALLY ENDS THERE'S AN EXCLAMATION POINT OR MORE LIKELY SEVERAL OF THEM!!!
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I'll stop with that, OTHERWISE I'LL GO ON FOREVER!!!