What's wrong with this, seen on a sign at a baseball game? "Put me in coach."
More than one of these
I laughed when I saw this sign, so "Nothing" is definitely wrong.
Consider "Put me in coach" and "Put me in first class." Without the comma, the sentence seems to refer to airplane seating.
Yes, you guessed it: A comma is missing, because the sign writer was addressing the baseball coach and requesting to be put into the game.
"Put me in, coach" is correct. Here's another example of this comma, called a comma of address:
"Give me a dollar, coach."