I’d left one of my bikes out in the sun, to make room to work on another inside the shed (I had already undone the seatpost clamp to use on the other bike but not yet removed it). Some hours later there was a loud fizzling noise and loads of smoke, didn’t see any flame, but it must have got pretty hot, and it stunk really bad too.
Moon Nebula rear light. Don’t leave bike lights out in the sun for hours on end!