Split infinitives:
"To staunchly oppose"; "To zealously argue" instead of the more correct "To oppose staunchly," "To argue zealously."
Ending a question with "at":
"Where do we stop at?" or (even worse) "Where's your mother at?"
Using "wit'" or "which" instead of "with":
"I'm coming wit' you"; "Peace be which you." (This seems indigenous to the New York area.)
In writing, using commas where semicolans should be used:
"I liked the cake, it was chocolate" instead of "I liked the cake; it was chocololate" or "I liked the cake because it was chocolate."