Chain slipping off middle ring.
 

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?:twitchy:

How is the FDER allowing that to happen?
Does it have exceptionally wide side plates or mounted too high?
Is it correct for the size of the rings?
What rear gears does it do this in?

Hmmm. I can't think of any other reason, unless the FD is setup so that the chain is rubbing in the middle ring and trying to shift.