When Good Discussions Go Bad

So ever have a really amazing discussion happening?  Every person involved keeps it respectful, and says 'what if' and never points to a specific person, or entity.  The discussion is kept general enough and focused enough that despite being a sensitive topic, no one feels persecuted or picked on or offended.  Then someone goes on a complete derailing rampage not really related to the topic at hand.

Ever had that happen?  Yea, it sucks.