I should prepare myself for that situation, really.
It has to be a song in standard tuning, because not too many people have a guitar in G-Tuning laying around and when I am drunk I don't want to tune it to G - would be a mess. So I usually just fool around with some country and blues tunes (Love Hurts!) if it's an acoustic - I sing like an angel, but only to my ears
Belgrade-Bucharest-Budapest-Brno