I decided to bomb it up a climb in granny gear. Changed fine to the gear, but my drivetrain started making a massive noise from the back, sounding like the chain was winding wrongly. I decided just to cycle up the climb anyway, and it worked fine, even though it was making a racket. Changed gear, went back to standard mode, and the bike carried on as if nothing had happened. Cassette looks fine, got no idea what happened. Anyone else experienced this with a Yamaha PW 2x10 Shimano XT?