I switched into loiter mode to do some loiter tuning. The copter started running away a bit, I switched back to simple/stabilize because I was near trees. The orientation seemed all messed up and cutting throttle all the way didn't even actually cut the throttle. The copter kept running away eventually into the ground. One motor was still spinning. I tried to disengage the quad (held down and left for 2 seconds) and the motor didn't stop spinning. I had to pull the battery to get it to stop.
Running 2.5.3 and I've had issues with not being able to cut throttle before, but I thought it was due to low transmitter batteries. Now I have new transmitter batteries and still have strange issue.
I had that problem with 2.4 or 2.5 I think. Alt-hold would start to go funny and switching back did not do anything. I was lucky enough to chase after it and catch it.
I stopped flying until there was a new version and it has not happened again.
Maybe post your logs so someone can take a look?
I think i had something similar, after coming out of loiter mode the throttle required to hover was way less. as a result my quad went up to 55m. but im not sure sure, may have just been bad piloting!
I have the brake setting enabled on my ESC's, so i could tell they are responding because at one point the motors stopped and the quad dropped and turned 90 degrees.
Normally the throttle stick sits at exactly mid-point - and there is very very little movement between rising and falling. After being in loiter mode, the quad seemed to keep going up even tho i was bringing the throttle constantly down all the way to about 1/4 throttle (which is just before the brakes normally come on). after that i went full down and it cut the motors and the quad fell out of the sky into a tree.
Logs and video are in one of my posts somewhere.