Earlier quoted context omitted.
> E.g. if I say "A is good for everybody, for X,Y,Z reasons" and I do the opposite of A, then the possibility of A really being good for everybody is somewhat lessened by the prior that "the person proposing A does not follow A". "It's good for you to not smoke. Now excuse me while I take my smoke break."
I'm not sure what is the example supposed to prove. That what I said doesn't always hold? Of course it doesn't. That's why I wrote about possibilities, and only said that the prior that the person preaching X doesn't follow X "somewhat lessens" the possibility that X is true. I never said that someone "preaching X and doing the opposite" proves that X is not true. If I had said that, your example would be a valid ref…
The correct answer is to independently validate the claims, or evaluate the merits of an argument on their own ground. That's tedious and often not worth the effort, and that's OK. We use heuristics all the time -- you don't ask your friend for his proof, because you use the heuristic of trust instead. But at least acknowledge that it's a heuristic and stop trying to claim it as a solution.