This article says that if incorrect words are used for the absolution then it is invalid. It makes no distinction between intentional and unintentional mistakes. If the penitent is aware of the mistake then he/she should confess again (for mortal sins).
The article focuses on the question of "Ecclesia supplet" and whether the Church would provide the sacrament when the form is invalid, and his answer is "No".
Many historical examples of invalid baptisms, confirmations, or ordinations would seem to bear this out. Ecclesia supplet does not remedy those cases wherein innocent persons bore the consequences of ministers making invalidating changes in sacramental form, and I don't think it does so for confession, either.
The article references an earlier article, which I was not able to find.
But short of that, God provides in other ways, too, right here and right now. He provides by giving us priests like Fr. Hoffman who will tell it like it is and alert penitents that such absolutions are invalid; He provides by telling these penitents that, while He knows these mistakes were not their fault, He still expects them to act on their knowledge of the invalidity of such absolutions and return to confession (assuming we're talking about grave sins, etc.); and I even think He provides by giving the faithful the confidence to contact their confessors, and if necessary their bishops, to inform them of serious violations of the gift that is sacramental confession.
Meanwhile, the rest of us need to be wary lest we assume too quickly that Ecclesia supplet will remedy serious mistakes in ministry just because they were not the fault of the faithful. Salvo sapientiorum iudicio.
Obviously, we are talking only about the essential words for absolution. "I absolve you from your sins, in the name of the Father, and of the Son, and of the Holy Spirit." A defect in other matters, such as not providing a penance, which comes up frequently in this forum, does not invalidate the sacrament.