At least 1/3 of the time. Often, I'll start to write an argument or an opinion, and then realize halfway through that the argument is wrong or the opinion is actually crap now that I'm looking at it in words.

Also, being from a conservative Texas town, I tend to have knee jerk reactions to more "liberal" stories and tend to want to blurt things out like "screw the poor people, it's all their fault". I don't really mean this, it just comes out from my upbringing. I usually delete those, unless I'm just really angry or have an insight as to why people feel that way in such-and-such a circumstance.