Does divorce occur if a man says to his wife: "If you do such and such, then you are divorced," and then the wife does that forgetfully, knowing that he did not intend divorce when warning her and wishes to retract his statement?

You cannot retract what you said to your wife, nor can you cancel it. If the wife does what the divorce was conditioned upon, then it takes effect as a divorce according to the doctrine of the majority of scholars. Nothing is binding upon you (neither divorce nor an expiation for an oath) as long as your wife did the action forgetfully, because she did not intend to violate it.