I always calibrate my compass but today almost found out first hand what could happen. Taking off from the same spot as last time and didn't calibrate. Auto-takeoff and within seconds I sense something is wrong. My indicators have satellites I got the safe to fly go-ahead, but no good. Phantom is drifting in air and my weak manual skills had to be used to land her. Find a patch of grass close to a metal fence, land right beside the fence with the winds blowing me around, safely landed but my propellers sliced the fence and broke. She didn't even tip over just propeller damage. Replaced and calibrated. Next flight was as smooth as always. Just a heads up, always calibrate your goods. NOTE: It has been brought to my attention by others on this forum that this has nothing to do with compass calibration, it was in fact a loss of GPS signal.