I've never had anything like that happen. Currently I ride with either Speedplay Frogs or Shimano SPD MTB-style pedals but I used to ride with Look's original design but they never did that either.

If nothing is obviously broken or missing, I'd assume a small stone or other bit of foreign matter got into the mechanism and temporarily jammed the rear clip. Clean them out and lube them thoroughly after a very careful inspection. Again, if nothing is missing or broken, you should be able to trust them on future rides.