You most likely got a glitch. I have had my heli just run up to full throttle and come up about 5 feet in the air and then fall. My buddy has his do it and it cost him a trip back to horizon for the heli.

It seems that the stock transmitter has issues if you get between it and the heli. It always seems that the heli will glitch if I set the transmitter down behind me then go to unplug it. It does sound more like you got one of the odd glitches than accidentally hitting the idle up switch.