I mentioned this one before, probably: his-self and their-selves.
When people use "I" when they should use "me."
"It happened to him and I."
"This is a present from George and I."
A similar incorrect use that really makes me cringe...and it's said all the time.
The incorrect use of he/him she/her
"Her and I went to the movies." instead of "She and I went to the movies."
Or worse, "Her and me"