What is the ruling on a man taking back his wife during her waiting period ('iddah) if the divorce was by mutual agreement, rendered irrevocable (bain), and neither of them intended it to be irrevocable?

If a man says to his wife: "You are divorced with a single, irreversible (ba'in) divorce," the divorce takes effect, but whether it takes effect as an irreversible divorce is a matter of dispute among jurists. The Hanafis and Malikis hold that it takes effect as an irreversible divorce, while the Shafi'is and Hanbalis hold that it takes effect as a revocable (raj'i) divorce. This is because a divorce does not become irreversible except for specific reasons such as khul' (divorce at the wife's instance with compensation) or a judge's ruling; otherwise, it is revocable even if the husband describes it as irreversible. This is what Sheikh Al-Islam Ibn Taymiyyah preferred and attributed to the majority. As for this specific issue, a ruling cannot be made without knowing the reason why the judge wrote the aforementioned wording, as there might have been a khul' or a ruling of annulment.