Saw it happen at MM a few years back and I’d always make a comment to them as I went past again further in to the lap
I think I’ve seen it happen at Mayhem too but never had enough evidence to say or do anything. You see a rider coming out of some bush where they shouldn’t be and realise that 20m further over is a doubling back of the course. I suppose he may just have gone for a piss or fallen asleep in a quiet sheltered bit though – seen that happen.
We got accused of cheating once at Dusk til Dawn – my team-mate went down to the bottom of the campsite to hand me up a gel (where the course came back near the campsite at half distance) and we got reported for doing an illegal change. Nothing ever happened though, I guess they must have looked at our lap times, decided that we’d started and finished the correct laps, there was no short-cutting, no changing so nothing more got said – in fact I think we only found out about the allegation from a comment on here.