According to ironmaidenismyreligion.net in 1984 and while Iron Maiden were in Poland, for their World Slavery Tour, they played at a… wedding reception!
“In 1984, before their show in Poland during their World Slavery Tourt the band had some time for a drink at a disco nearby their hotel. Getting in they realized that there was a wedding reception. The groom and some other guests identified the members of the band and asked them to get on stage and play a song. To their surprise Iron Maiden agreed! They got on stage, but instead of playing their own songs they played a few covers including Deep Purple’s “Smoke On The Water!”