I know I posted this in the mechanics forum, but I don't know how many of you go over there, and I strongly suspect my being a Clyde had a big part in this mechanical (structural?!) failure, so I'm sharing it here with y'all for instructional purposes.
This happened to me as I went to take off from the left turn lane at a busy stop light, early in the second day of an overnight, two day bike ride. Had to catch a bus into the next town, as the town it happened about two miles outside of didn't have a bike shop, at all.
I know my weight is a factor here (280 lbs), but I suspect the pedal extenders also probably put some additional stress on the crank in a way I guess it wasn't really designed for. Nonetheless, I never even considered that such a failure was a possibility prior to this happening to me. I guess I just assumed that a cast piece of aluminum was basically unbreakable in this context.
To be fair, in addition to my weight, I ride for nearly two hours a day, seven days a week over rough roads, and occasionally end up hopping off a curb during my commute, so I suppose that poor crank has been subjected to continuous vibration and occasional severe stress.
PS: Ride report on the trip follows when I finish composing it!