Doing a Harry Potter

Leaving the girl you have a crush on to your best friend for instead being able to make out with his sister.

A: "Did your hear that John is now dating Ryan's sister?"
B: "Well, Ryan is in a relationship with the girl John had a crush on, so I assume John was doing a Harry Potter."

Doing a Potter

adj; To do a disappearing act. Going AWOL/MIA from work in particular leaving your colleagues to pick up the pieces.

Bob: "Morning Trace, nice to see you"
Tracey: "Alright Bob, 'ave a nice holiday. You looked very tanned!"
Bob: "It was lovely Trace.......where's Peter?"
Tracey: "He's got his invisibility cloak on Bob..*sarcastic smile*... He's been doing a Potter since last Wednesday. Not even called in you know?! The cheek..."