I had something kind of strange happen to me that I thought worth sharing.  I was finishing a short & uneventful flight around my home with my 3" gecko and had a light crash-landing where the quad bounced off my fence before hitting the deck--not all that hard really.

I'm reflexive about disarming when I get down and/or when bad things happen, and I believe I disarmed... the quad came to rest and went still; no props in motion.

When I went to pick it up, motor 2 alone spun up fast & I got bit as I pulled the lipo plug to kill it.  I was lucky, I lost a little patch of skin, but nothing horrible.  More than a little surprised and indignant tho! 

The possibility exists that I failed to disarm.  This it about the only time I've ever experienced an unexpected spin-up, when the quad otherwise appeared safe.  (I faintly remember once having a hard time disarming after a crash or something, but that was a long time ago and a very different quad)

I'm flying 4.5 RC2 & understand the risks of flying pre-released firmware, but frankly, I suspect the FC or ESC simply took enough of a knock on impact to scramble it's brain temporarily.

I removed the props, plugged into bf configurator to check for life: calibrated accelerometer, checked gyro traces, spun up the motors one by one & everything seemed OK.  I considered re-flashing for good measure, but didn't (GPS is on UART 1, so need to disconnect 1st).  

I went out and made a short shake-down flight today and everything seems fine. 

Yeah, so be careful, especially after a crash. 

Poll: Has anything like this ever happened to you?

I've seen impacts temporarily scramble electronics before. Could just be a one-off event where the flight controller decided to output a random signal to that signal wire. Even when disarmed, the ESCs are all live, so they'll still obey whatever signal they're given.

I had one bite me when I first started, but I was doing failsafe tests while not wearing a t-shirt and it flew up and chewed up a nipple. No blood drawn but it hurt like crazy. Thankfully only a 2S whoop, so not the worst ever.

Never heard of a disarmed quad going nuts, but there are possibilities with anything powered up. If you get a short in the wrong place, right through to high energy interactions (read about, never seen, but flying in proximity to certain radiation sources can screw with electronics). So if you bump it on it's head and it does something unexpected, well, if you bump a human on the head the same occasionally happens as well.