Have fallen off a couple of times in cleat related incidents. Only ever due to my stupidty at leaving it too late when coming to a stop. Last one was next to a ditch ended up falling into it in a pile of nettles with my bike on top of me. Since then I have always decleated much earlier in the stopping process.
I can only recall one incident where my cleats were the source of problems in 20+ years of using them. It's quite the story... Getting out of the cleats was the least of the concerns that day...
A buddy and I were down in Moab for the weekend. The afternoon of our arrival was a typical summer day -- ~100 degrees. We rode up Poison Spider and (IIRC) came back down Poison Spider rather than the portal. It's a fun 1000ft climb over an old jeep trail. The step rocks are fun, the slickrock is fun. The sand sucks. You wouldn't think it, but it took quite a bit out of both of us.
The next morning we started the grind up to Porcupine Rim (back then shuttle services didn't exist). In all it's a ~3000ft climb over ~13 miles. It's a mix of asphalt, washboard, sand & slickrock for the first 10 miles, then a wickedly fun technical climb up the jeep trail to the overlook over the last 3 miles. I started cramping at some point during the initial 10 miles of the climb. By the time we got to the overlook it was well past 100 degrees and my legs were regularly cramping during any kind of burst climbing (and with the step nature of the jeep trail climb, that's often). But you know, I'm not the smartest guy, so we kept climbing up to the overlook.
Happy as hell for the climbing to be over, we rested at the overlook and eventually started our descent down a wonderfully fun jeep trail for several miles. At some point on the way down my buddy spilt most of his extra water while transferring it to his camelbak. We split mine. By the time we hit the singletrack we were both out of water, probably with diminished decision making capabilities. We eventually hit the 5 miles of amazing singletrack. Near the end of the single track we were both totally fried and plopped down in a shallow pool of water and rested for a very long time.
After a while we heard someone calling for help from up the trail. There was a group of 3 riders, one of whom was barely able to walk with her bike. We rested her in the pool of water for a while, then proceeded to walk her out for the last mile or so to the road. My buddy rode ahead to get water at a spring a few miles down the road.
We got her down to the road about the time my buddy got back from the spring with water. Flagged down a raft shuttle bus who took the lady to the hospital. Asked if any of us needed a ride. Nope. It's just 4 miles of road work to town, no big deal. With fresh water, from my buddy, we started the road ride back to down. Asphalt radiates heat. I have no idea how hot it was on the asphalt, but the official temperature that day was 106. Every time I felt bad, I drank more water. At the intersection with the main road a mile outside of town I stopped to let a car drive by, leg cramped and I promptly fell in the middle of the road unable to separate my foot from the pedal of my bike. Quite embarrassing.
I blew out all the water I'd drank in a parking lot just outside town. I spent the rest of the night and many hours the next day cramping every muscle in my body. Arms, legs, chest, eyelids, toes, fingers, you name it, it cramped. Absolutely miserable. Fall asleep, then wake up in searing pain from a cramp. Rinse, lather, repeat.
A week later, in similar conditions, two young guys from Iowa died on that ride under similar conditions. They didn't carry enough water and (presumably) stayed on the jeep trail rather than taking the turn onto the singletrack which leads back down to the road (guess they didn't know the route). The jeep trail peters out at a cliff a ways after the turn-off. They were found on a ledge -- apparently they started climbing down the cliff and reached a point where they couldn't go up or down and died from exposure.
That winter I nearly got frostbite on the same ride... But that's another story....