Bike setup, 8 speed with triple crankset. Middle ring are new and 10 speed, chain is 8 speed and fairly new. Twice under load today, the chain basically lurched forward and dropped from the middle ring to the small ring. The chainrings/chain difference should not matter on paper. Any ideas? Even with a worn chain and new rings, would forward load be so much that it would actually cause the chain to come off the ring??? Weird. Im not ruling out user error and accidentally hitting the thumb lever to down shift but im usually good about not doing so. ideas?