Using M.P. verson 1.3.62 and Arducopter firmware version V3.2.1. I attempted to do a motor/compass calibration by installing the propellers upside down and running the test in my basement. After getting to the calibration page and clicking on the start button, I armed the motors and started to throttle up - gradually increasing thrust until I reached the maximum. But then I noticed that there was no graph on the calibration page, so I tried to throttle down the motors but they continued to run at full speed. I tried everything I could think of - tried turning off the transmitter, but remembered that I had the groundstatioon fail safe enabled and set to RTL. I then went to the Motor tst page on M.P. hoping to shut down the motors by clicking on the All Motors Off button, but M.P. said that the command was refused.
I finally had to resort to laying on the floor and carefully reaching for the battery plug with my arms underneath the spinning pros and unplugged the power. Really scary!
Later, I attempted the same thing, but this time after hitting the start button, I made sure that the graph was starting and I throttled up the motors without arming - the test went fine, and later I flew the copter with no incident.