I figured we could start a thread about common grammatical errors that pop up all the time and annoy the graders. I'll start us off with two classics.
The Comma Splice
You are not allowed to join two independent clauses (sentences) with a comma. Use a semicolon or period instead.
WRONG: Bob was hungry, he hadn't had anything since breakfast.
RIGHT: Bob was hungry; he hadn't had anything since breakfast.
RIGHTER: Bob was hungry since he hadn't had anything since breakfast.
Names and Commas
When a character is talking and he or she mentions someone's name, that name must be set off with commas. Must. Both sides.
WRONG: "Hey Bob how are you?"
STILL WRONG: "Hey Bob, how are you?"
RIGHT: "Hey, Bob, how are you?"
If the name occurs at an end of the sentence, you only need one comma:
LEFT: "How are you, Bob?"
RIGHT: "Bob, how are you?"
Come on in and let's have ourselves a nice kvetch.