someone else's hangover
This is when you wake up in the morning feeling like you really tied one on the night before even though you didn't have anything to drink. Through some sort of karmic mixup, you woke up with someone else's hangover, and they woke up feeling like a champ.
me: Ouch. My head's really banging this morning.
you: Well, you didn't have anything to drink last night - must have someone else's hangover.
you: Well, you didn't have anything to drink last night - must have someone else's hangover.