On the long list of ineffective persuasion techniques is telling someone "I told you so", after someone else realizes there were incorrect. It takes several forms, many of which are quite subtle, and it's possible to say it without knowing you're saying it.

If your aim is to tell the person they were wrong, then you're being redundant. More than likely, the person already knows they're wrong, so it's just insult on top of injury. And if your aim is to get the person to listen to you again in the future, it's still a blunder, because criticism is rarely a good way to gain credibility or influence over another person.