A few years ago my old band toured with a Canadian glam rock band called Robin Black.
One night, early on in the tour, we played one of their songs in soundcheck for a laugh. They heard us playing and decided WE should play it as their encore.
Every night from then on they'd finish their set, we'd come on, pick up their gear and start playing their most popular song, often to the crowd's bewilderment. Then the band would gradually join us on stage to finish the song. After that we'd give their gear back and they'd play a couple more encores and then leave.
That was a fun tour.