Not quite a session; but some years ago, while roadying, the girl I was supporting did a gig in the Mason's Arms, Hadfield (the one featuring "Live Indoor Rugby" in the League of Gentlemen.

As I set up on the stage, the projection screen behind it was showing "Casablanca". (They had been showing football earlier in the day, and not turned the TV system off.)

As the time came to start the gig, "Casablanca" was still on. I asked the bar staff to turn it off. They could manually turn down the sound, but couldn't find the remote control to turn it off. (Pulling out the plug would have lost all their carefully entered tunings and settings.)

So, the plucky girl did her first two songs while behind her Ingrid Bergman silently asked the pianist to "Play it again for me, Sam." The landlord then found the remote, turned off the film and instead unearthed that awful backdrop of glittery strips which seems to be de rigeur.

Personally, I would have preferred to keep Bogart and Claude Rains on, albeit silently.