Mine is people booking return air too early.
Some examples of my pet peeves:
1) I know the ship docks at 8 am and I booked my flight for 9:30 am and the airport is an hour away, can I make.
2) I tried to sneak booze on board and they got me--I'm mad at them and won't sail them again.
Lets hear yours.
But remember this is just for fun.