One of my biggest grammarpeeves is the misuse of there/their/they're. It grates on my nerves like crazy.
I recently typed up a story and wasn't able to proof read it for a few days. When I DID proof it, I found that I had used THERE instead of THEIR *3 TIMES!!!*
*hangs head in shame*
That, my friends, was a total typo. And I do it frequently, but tend to notice as I type it that I just typed there instead of their. Ugh. And so anyone who read it before I fixed it probably cringed, and thought I was a dummy.