You know, we all make mistakes. Maybe we react too quickly to something said, buy something they really could have done without, or perhaps judge someone too negatively. So, what can you do when someone paints themselves into a corner?
First, as they slide into the mistake, and you see it about to happen, you can encourage them to take their time. You can mention things they might want to consider, while letting them make their decision. It IS their decision.
Then, when they discover the truth, and they have to look in the mirror and say “Hey, that was a mistake”. That is bad enough. Don’t make it harder by being the one to say “I told you so”. Let them save face; it will then be easier for them to choose to correct the problem.