It never happens on rough ground because I expect it, it's always a rogue little pebble in the middle of otherwise perfectly smooth ground.
About 7 years ago i switched from the shoes i usually bought to another style that had a significantly lower heel. I'm skating to work, get to a busy intersection, and I put my foot down to stop, transferring my weight to that foot. My muscle memory expects my foot is soon to be planted but it's not, and I just fall straight on my back, and shoot my board out into traffic.
The WORST feeling was doing the exact same thing a few days later at the same intersection.