Riding up a bridge this morning in a cyclelane, light traffic and a guy in a hiviz builders coat up ahead. I soon catch up with him and he hears my freewheel and very kindly pulls over to the side to let me pass. I check behind and go wide, say "thanks mate" and hammer down for the downhill. Only problem is when I click in the bigcog and my chain comes off and wraps itself right around the pedal and crank! Completely locked up. Had to take a detour and get off and fix the thing. That could have gone soooo wrong!