Well, first you meet the pavement real fast. And then you need to figure out how to get home with only one functional pedal (shoe on the stump doesn't work all that well and to bystanders you look darn weird.)
So, this crank was a Sugino Mighty Tour that I had set up with a single 40t ring on my IGH commuter. It is early 80's vintage, but probably had less than 4000 miles on it. It was never damaged and I doubt that my 60 year old legs have suddenly developed mega power. So why did it break? Other than the groove, I see no real stress points. Microscopically, the fracture shows 2 colors to the metal, so I think there was a micro-crack half way through it that finally gave up. Does anyone think this may have been a manufacturing defect? It's got me scratching my head.
I took my dive when pedaling up hill (of course) and luckily there was no traffic around. It was remarkably fast - I had no time to think about what was going on.