I had one experience with this that was an absolute horror show. Only time I'd ever been to Brighton England, and at a really important headlining gig. 2 1/2 hour set, guthrie govan was joining us for a couple of songs. The bathroom was 2 flights up in a packed venue, and the only way I could get to it was through the crowd. The urge kicked in during the 2nd song.
If I lived it again, I'd have peed in my pants. Was stupid for me to ruin that evening for myself by holding it in. When we were done we got an encore. Everybody had to wait for me.
Found a coupla pix of the fond memory: