If you knew a manager is 100% wrong about something how would you handle it?

There are two scenarios: First, this manager is open minded then I will analyse the case thoroughly, and examine it from different points of view.
Then I will meet him in his office and will express my case without pointing that he is mistaken but just suggesting to do these changing because of these benefits and implications.
secondly, if this manager is autocratic, then mostly he will refuse to listen to me and will turn the conversation into personal dispute, I will prefer not to confront such managers and check what I can do to fix this problem without face-to-face confrontation; just indirectly.
like searching for a solution then trying to pass this suggestion in short to your direct manager (if he is not yours) and don't try to link your suggestion to his own wrong decision.

I always believe that there should always have an open communicaiton between you and your manager.
If he is100% wrong and it affects your work to perform well, you can inform him on what is wrong.
But saying that he is wrong should always be back up by facts.
Discuss with him what are the pros and cons of his action and your suggestion.
You have done your part and it is up to him/her if he is open minded.
If it personal and not work related, better not to talk to him because at the end of the day he is still your manager.

It depends on the situation and the personality of the supervisor.If i know my boss is100% wrong about something, never tell him he's wrong.
Never.
Whatever he tells me to do, I smile and say "okay." Then if I know a way to get it done that will work, I do it that way, give him the results he wants and never tell him I didn't do it the way he told me to.
He got the results and is happy.
I saved myself the stress of being yelled at and gave him what he wanted

By telling him the cons and prose of the decision and letting decide him self.
After all he is manager and he is accounatable