we just encountered an unusual crash at our hexarotor. In the middle of the flight, the autopilot "froze" - stopped responding, stabilization stopped, no telemetry was received and even more we had no radio control, so the kill switch didn't work.
After the freeze, the airframe tilted on the side and crashed into the wall.
However, the motors kept running all the time (after the autopilot froze and even after the hit) so they had to be manually unplugged.
As ESCs run only if they receive a valid PWM signal, the only explanation I have is that the main loop halted for some reason, and only thing which was still running was sys_timer interrupt which controls the PWM signal generation.