Yeah, everyone who's ever used clipless like pedals probably has at least one "Tombay" incident. Personally I've only had that happen once that I can recall but my gut is telling me back in the early years when I bought my first SPD's it likely happened more than once. For me the major cause of the Tombay falls was not forgetting to unclip but trying to unclip from my cheap SPD's and not being able to disengage from the pedal. The reason for this in my case was the cheap Nashbar and Ritchie SPD's I was using on my bikes back in those days. When I finally switched over to decent Shimano SPD pedals it pretty much eliminated any problems with the pedals. After that likely the only other time there was ever a problem disengaging from a pedal came because the cleat on my shoe was packed with mud. I pretty much now abstain from overly muddy MTB rides.

When it comes to disengaging from your newly acquired cleated pedals, it makes a big difference if you also have good shoes in which to mount the cleat. Setup is important too. If the tension is set too strong you will have times when you have problems disengaging from the pedal. Too loose and your foot will disengage at times when you don't want it too. Once you get dialed in to your pedals you're good to go but that period of time when you are trying to figure out the best adjustments is the time when you are likely to have problems with engagement / disengagement from the pedals. In the long run though it's always worth it as long as the falls you take are the slow-motion types.