To a philosopher, there likely comes a time when the error in another's crazy

To a philosopher, there likely comes a time when the error in another's crazy ideas -say, at a party or dinner- can be so apparent as to invite some criticism. What's a good moral position on whether to correct someone's logic when it's uninvited and suspectedly unwelcome? Idea eg. UFOs, 'crystals', ESP, conspiracies, etc.

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