How would you deal with a manager if you knew he is 100% wrong about something?

Don't tell him that he is wrong. Just make things and related issues show him indirectly that his is wrong.

I have learnt many things in my life, this is one of the list.

During my early days of career, I used to be straight forward and up front saying 'this is wrong' and used to dig my heels in the ground.

After a while I understand this does not work  in industry.

At the end of the day, manager expects me to follow instruction.

Now I deal with alike situation using3-point action plan.

1. Mention the wrong

2. Document somehow that could be a reference for me in future

3. Follow the instruction, even if this is wrong

This may be a political move (you may say that) but believe me I did not find any better in my experience.

At the end of the day manager does has accountability of his/her actions.

I dont directly tell to my managers that they are wrong. I usually throw a lot of question regarding that. Indirectly i tell them. I dont want them to see that I'm checking on what they do or feel shame because i notice some wrong on what they do. except if they ask me directly thats the time i will tell them, but in private conversation.

Right or Wrong is individual's perception. What I believe if someone knows what he is doing is wrong why won't he do it  , he does it because he believe it to be "Right" similarly we believe something to be Right which may not be right for others ...so instead of being Judgmental it is always better to ask your manager the reason and logic behind his act/action may be that will lead you to some conclusion ............

I think it'll need a tricky way of dealing try to ask him to clear his point this will open a space to discuss his thoughts freely & youll be able to ask as many questions you want , through these questions you can make him figure it out .and rejudge his own choices.

For me, this is a tricky question since the way you tell your boss that he is mistaken will depend on your relationship with him.

 

If, in the beginning, you have been perceived by your boss as a confident and bold subordinate, telling him right frankly but respectfully will be the best way to do it.

 

On the other hand, If you have been perceived as a subordinate who is less confident and always "blindly" follow instruction from you boss, then you have got to make your brain cells working and think of a better way to do it. A exemplary instance would be talking to someone who is a confidante of your boss and asking him to point out his mistake for you.

that's why we have meetings , simply meet and discuss it

A manager is one if he/she accepts contructive criticism and who's receptive to new ideas. As long as you raise an opinion in a courteous manner, there shoudnt be any issue. As the saying goes: Its not just the content of what we say, but how we say it.

If only you knew that he is wrong with any particular decision, you should not worry much because right or wrong is different to each person but if your colleagues too feel the same its best to tell him personally in an indirect polished way with out hurting his pride.

Good Luck 

Well,in my openion in case of any disagreement communication always plays a concluding role.I believe that no one can be100pec right or wrong.its only the way we see every thing.Instead of confrontation its always better to try to convince other person or get convinced to keep the work environment healthy.

Confrontation and bold frankness is your worst bet. Diplomacy can be a good choice to get him to see things right.