Originally Posted by Al Skierkiewicz

What you describe is a classic case of miscalibration. Remember that the controller when it is in calibrate, determines the full forward and reverse of the joystick travel and the at rest position and then stores them when the calibrate button is released. If the robot drives forward with the joystick and then slows down but doesn't stop when you release, try recalibrating. Failure mode on the controller could be no movement in either direction if the FETs go open or full power in one direction if they FETs short. If all FETs short there is a brief period where there can be smoke, fire, arcing and/or the sound of cracking plastic.