In university one guy passed out and got an eyebrow shaved (Call him "B").
To get even, the guy went to the nearby bathrooms and removed the taps, then went up the the guy who shaved him (call him "A",) and rubbed some junk in his hair. "A" was also known for being somewhat narcissistic.
Resulting conversation:
A: "WTF was that?"
B: "Remember when you shaved my eyebrow before Christmas?"
A: "Yeah, so you rub gel in my hair?"
B: "Not gel..
A: ????
B: ..Nair"
A:
Nair takes a couple minutes before it works, so by removing the taps it took "A" a while to find somewhere he could rinse the Nair out. Too long.
Sometimes payback is a B!tch.