I've been waved through several times in France, and never had a problem leaving. Most of the time they don't look for the entry stamp anyway, but I've had pages added twice and have oodles of stamps. Perhaps they can't be bothered.

OTOH, when passport control in Alicante, Spain couldn't find my Dad's entry stamp, we almost missed our flight. I'm not sure what would have happened if we hadn't found the stamp.