Well, that was educational.

I had my optimal routes for Yonge and St. Clair planned out, and before I left, I went though my usual check of tires/brakes/seat. What I didn't do was check that my left crank was firmly affixed to my bike.

I thought my left foot was slipping oddly on the pedal on the last stretch of Davenport before I turned on to Greenlaw, but after I walked the bike up the (short) killer hill (just ahead of some pretty fit looking guy on a mountain bike, so I didn't feel too bad), I hopped on again to start cycling the side streets and immediately lost the crank. No sign of the nut anywhere on the street, so it may have actually fallen off on Davenport.

So it was a pleasant walk home, where I considered all the places I could have lost it instead -- booting through the left lane with a bunch of cars on Old Weston waiting to make the left on to Davenport, spinning madly uphill through one of the underpasses, or, even better, anywhere partway through tomorrow morning's ride to work.