amishbill found an issue on the SP31 V2.0 lights, the tactical-style model which is almost identical to a Fenix. We’ve only tested two, but they both had the same issue — the tail switch sometimes gets stuck and doesn’t want to turn off. It clicks normally, but whatever part makes the electrical contact inside, it can get stuck until it’s clicked again a couple more times or until the light gets knocked against something. Sometimes it’ll even just fall into place on its own, after a short delay.
On the one I tested, sometimes it’ll work fine 50 times in a row, and sometimes it’ll fail twice in a row. Overall though, it seems to get stick about 1 out of every 25 times I turn the light off.
I’m guessing that it might help to put a bit of lube inside, since that’s a common thing for some types of switches. Not sure though.