I was delivering some stuff to Josh Groban's dressing room once, not expecting to see him (thought he was out), when he opened the door and asked if I could show him where somewhere was. That was unexpected, and he was lovely.
Shared a lift in a hotel with Motley Crue.
These were situations where I knew they were doing shows, so maybe don't count, though I wasn't expecting to encounter them at that moment.
Bumped into Maisie Peters in Edinburgh a couple weeks back... again, knew she was in Edinburgh for a show, but didn't expect to see her there.