Perhaps i'm necroing these threads considering the 3 week rule, but equally considering the lack of activity here I don't see it as a big deal.

So, first note...
I disagree with MDK's change, as it subtly moves it from being a matter of "When proving something is detrimental" to "Justify why you should prove it". Which to me, kinda changes the default, as I see proving things to be intrinsically a good thing, rather than a bad thing. Why? Well - proving something means demonstrating the validity of whatever is being claimed, and demonstrations is a universally received form of information, rather than a perceived conceptual one where our limitations as human beings often have the words in which we attempt to communicate things be stooped in bias associations, which is a failure of communication. And healthy communication (talking over dinner) is just plain ol' better than detrimental communication (talking over military trenches), war and such can be regarded as a failure to communicate entirely, arguably. . 

My answer to the actual question would be "When it proves detrimental to the well beings of others, without a means to participate resolve on both parties". That basically means, anything beyond "Oh i'm a bit upset from being proven wrong, I may need to question my identity and actions in reflection to this new world view now proven" .