Unfortunately, the GFCI is tripping occasionally when the bathroom vent fan is turned "off". It never trips when turning on the fan, only when turning it off. And, it may go a few weeks before it trips, and then suddenly trip three times in a row. Then it'll work fine again for a few weeks.
I suspected a bad GFCI, so I replaced it with a new one, and also replaced the switch that controls the fan. It doesn't seem to trip as often now, but it tripped for me again last night when we were visiting.
The only remaining item seems to be the fan itself, but I'm curious if there's some other possible cause I might be overlooking? The fan has been installed for a few months now, so I probably can't return it at this point.
I thought maybe condensation from moisture draining back through the vent pipe (it had to be routed up over a beam to get out through the side wall). But, the tripping doesn't really seem to be related to when the shower was used recently.