Thanks for all the comments. It's very helpful to hear from everyone. I hope I haven't worried anyone else with my "horror" story. As one poster said, the SPD M324's are pretty cheap, but it was the first unintentional release I've had with them since buying them two years ago. Plus, the cleats on my shoes are pretty scuffed from walking around in them. Add the fact that I try to sprint hard as I can in traffic to keep some space between me and the cages, and so I may have twisted my foot slightly. The combination of time, wear, and unconscious twisting probably summed up to make this inevitable.

That being the case, part of my solution is to make sure the next pedals I get are not likely to make mincemeat of my foot when I (inevitably) unclip again. The M324's have a platform side with a serrated metal edge to grip your shoe, and I think this was half the problem. I should have committed to one type of pedal rather than try to compromise between two. Out of all the pedals, the speedplays, and especially the frogs, seem to have the fewest edges. I haven't seen one in person, so is this the case?