I know of someone on another forum who became concerned that comments made in jest would be read as serious and true by his wife. For those whose spouses/mates read a forum, some comments could indeed spur a rather unpleasant discussion -- probably a rare event, but sometimes the banter could go that way.
It also became apparent that one new member was disturbed and was determined to disrupt the forum -- finally had to be banned. Members have since seen this fellow in multiple places.
If I have a good enough connection with someone on a forum, any really personal info would be shared by PM.