What is a Poisoned Cue?
Cues are a signal to the horse that he has an opportunity to earn a reward (food, play, attention, do another behavior he likes to do etc) by performing a behavior. When a correction, punishment or aversive experience occurs after a cue (even inadvertently), the horse will stop responding to the cue since it no longer is a reliable predictor of good things. That cue can now also mean bad things may happen. This is known as a poisoned cue and results in the horse responding inconsistently to the cue and weakens the behavior.
If the horse is trained using correction, the cue (actually called a command) is a reliable predictor that if the horse does the behavior, he will avoid a correction for not doing the behavior (his reward for doing the behavior).
How do you Retrain?
Retrain the behavior from the beginning (should be much faster this time around if the horse already knows it) and teach the horse a different cue. Since the new cue has no negative association with it, he will respond willingly, once he knows it. Always make sure he is only rewarded for the behavior when performed and never punished or corrected.
Here is a horse that sees the halter as a poisoned cue. You can see his reaction at 0:38 seconds when he sees the old head collar on the rail, then at 1:11 when it looks like it will be put on him. By starting from the beginning and retraining (I am assuming they used targeting) with a new halter, they were able to get him to be comfortable with it.
Video from OakfieldIcelandics's channel on youtube.com