wvmarle:
That just doesn't make much sense as you always know where a servo is: at the position you tell it to go.If it's not there, then there's a malfunction of the motor, or it's not strong enough. Either way that's a design error, and should never happen. Therefore having a pot to read back the position of a servo is unnecessary.
That, or your description of the problem is flawed.
I will be turning the servo off to move it by hand