I've been using SPDs on some of my bikes for almost 2 years. I have actually experienced this before, on other sets of SPD pedals, but I've never fallen because of it though. I can only duplicate it with a monster (pretty much max for me) effort, and even then, less than a quarter of the time even if I'm trying. This level of effort never occurs during normal city riding, and not even in many races. I was doing a specific training drill in which I was trying to produce the instantaneous maximum possible torque.

I have put many similar efforts into my Look pedals and never once pulled out.