I'm going to laugh at my girlfriend when she slips on a banana peel
Why would you laugh at your girlfriend for slipping on a banana peel?
I think it would be funny.
Seeing someone slip on a banana peel is quite funny, but you shouldn't laugh at your girlfriend if she does so. Laughing at her isn't very nice, and it's quite painful to slip and fall. You should be more sympathetic towards her, otherwise she might break up with you.
I'm sure she'll be fine and we'll both have a good laugh over it.
Your girlfriend could fall and hurt herself if she slips and that wouldn't be very funny. Partners shouldn't laugh when one of you has an accident and is injured. Think about how you would feel if the situation was reversed.