I have a couple that it's difficult to choose between.
We were booked to play in a night club on a Tuesday night. We didn't get very many Tuesday night gigs so took it. When we turned up the place was empty apart from bar staff. However we were assured that a coach party were on their way. Sure enough a coach eventually pulled into the car park and disgorged a crowd of middle aged/elderly folk. We thought it a little strange that most of the women seemed to be clutching plastic bags. They marched into the room, sat around the edge of the dance floor and got out their silver dancing shoes. Yes, we had a party of ballroom dancers! I'm still quite proud that we managed to blag our way through an evening of quicksteps, waltzes and foxtrots - we all had "showband" experience. Certainly not the night we had anticipated!
The other was the time that we played at a wedding. Towards the end of the night there was a bit of a kerfuffle. It turned out that the brother of the groom had beaten his father unconscious. Police and ambulance were called and the whole venue was in lock down for a couple of hours.
Oh, and the wedding we did where the only people to turn up were the bride, groom and maid of honour! Our trio were in danger of outnumbering the guests!