Have you ever had your timing chain "fall" off during a ride?
We did a rather difficult century on Saturday, described in the link below, and just as the sun went down, we rode over something which neatly removed the timing chain. We have no idea what we rode over, although by the way it sounded and the way it hit my foot, my impression was that it was a branch. Next thing we knew, the chain was dragging.
I was afraid, at first, that the chain was broken. But it was all right, and after a couple tries, Rowan was able to get it back on again so that we were reasonably close to in-sync again and we finished the ride.