My most recent was a few months ago. I had just left the house and was coming to a stop behind a car at a traffic light with a car behind me and no where else to go. I couldn't get my foot unclipped and I had to stop or hit the car in front.
Then all of a sudden I realized I could un-clip the other foot and all was well, with no embarrassing impact to the car or fall. <grin>
Wound up being one of the screws in my cleat had become loose. I guess since the shoes were maybe as much as ten years old I should have checked the security of them a few times since putting the cleats on.