That's an acute approach.
Probably works better than telling them outright why they are wrong. Can't tell you how many times I've warned people of dumb ideas and had them pissed off at me only to have to bite my tongue later to keep from spouting a useless "I told you so" after they go and ignore me and it turns out badly.
I personally like the Devil's Advocate approach; help to see better what a person's ideas really are, and at the same time challenge your own.
Reminds me of a kind of similar problem here. I always give answers only hinting the correct answer, but then somebody comes and gives 100% solution...
How many times I just did that to laugh at them in the past and more recently to not play the loudest voice wins the argument? I mean c'mon I need a respirator to sleep. -_-; Not like I would spend all my air and saliva on stupidity. One calm warn and... Well, let the Lord decide.
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