Well, I've never seen a bathroom with two doors, either. No, wait, that's not true. Only in college dormitories with shared bathrooms - but that's certainly not a typical residential environment. (-:
It's still most likely to be a bad switch given its location (wet bathroom on the side of the medicine cabinet) and the symptoms the OP described. A switch like that may not be well-sealed and after years of operation with damp fingers crud may have made its way into the switch internals.
I'd be even more certain of a bad switch if the load was more than 25W - the closer the switch is to something hot, the greater the chance for failure. When I used to use 300W photofloods in the studio, the cheaper lamp holder switches would fail with regularity - which is why I think it's a bad switch from what the OP described. Obviously the greater the current, the greater the potential for corrosion and material failure.
I'll bet the "feel" of the switch has changed. Whenever I've had a toggle switch begin to enter a failure mode, the switch feels "sticky" or "mushy" and doesn't have the right "snap" to it.
-- Bobby G.