I was on a train once which was detained outside Clapham Jn for almost half an hour due to the weather, during this wait at least one handle was pulled (it got really warm). I've seen a call for aid hit by accident once, as well as one time a passcomm which refused to reset and shut the line down for half an hour in the peak.
I've come very close to hitting one myself after the toilet door handle got stuck; but never had to use one (yet!)
In all purposes, if I can find the guard rather than stopping the train, I will.
The exception being if, on slamdoor stock, I find a door on the catch; or on any stock if I see someone trapped or in danger, I will immediately stop the train regardless. I would rather stop the train then explain myself and get a telling-off from the guard than not stop the train and see something dodgy happen that I could have prevented.