A phenomenon that can only be put down to self trickery.
When you go out one night and decide not to drink, everyone else around you is drinking or already drunk to the point of obliteration. You go about your night sobre, maybe get caught up in the present drunken vibe, retire late / early morning - with everybody else.
You wake up at an un-godly hour of the day due to the late night - BUT, you feel shittier than times when you've been unrealistically drunk.
It is clear that the brain is either so used to dealing with excessive alcohol - blood ratio on a saturday night, it brings you down the next day regardless, OR you are such a lightweight, that just being around alcohol is enough to bring on a hangover.
Personal experience shows the latter observation to be false.
Dude 1: Dude, I feel WORSE today than most hangovers .. W..T..F!?! I was sobre all night!
Dude 2: Placebo Hangover dude *shakes head* .. my condolences