Say that a person comes to confession and the priest gives bad advice or otherwise makes a major mistake during confession. To give a very clear example, say that a confession went like this:
Penitent: Father, I've murdered someone.
Priest: That's not a sin, don't worry about it.
Penitent: Really? Thanks Father!
Only after reflection did the priest realize he misheard the penitent and didn't realize he was confessing a mortal sin. The dilemma here is that he is under the seal of confession, but he made a major mistake and now the person thinks that it's okay to commit a mortal sin.
A more subtle example might be a priest hearing a confession that he was unsure was sinful, but after reflection, research, and prayer, he realizes that his spur of the moment words was incorrect.
Would the priest be allowed to talk to the person after confession, or during future confessions, in order to correct himself?