A fair few times. Sometimes I start writing with the knowledge that more than likely I won't actually send it but I just find it therapeutic to write it out.
It's usually on topics of a serious nature when my opinion may go against the typical consensus and just can't be bothered to deal with how hitting post may go. As in (and has happened when I have hit send in the past, only one time I can [possibly] think of here but I mean just in general on the internet) a fair amount of people can't help but box people in to why they think that other people must have that kind of atypical opinion. "OK you think that so it must be because of X Y and or Z, now let me explain to you why those reasons are stupid and why you can't have thought about this in great detail"...actually no my opinion is not because of any of those reasons and I took the liberty of preemptively explaining why someone may (fairly) make those assumptions but my actual reason is something different...but thank you for ignoring that completely to go ahead and box me into your world view of the only reasons people can hold that kind of opinion so you can come off trying to sound clever and not at all condescending!
Then there's the whole if you call out someone (clearly) being condescending towards you because you challenged their ego that kind of person will likely play innocent and try to make out that you're angry (and therefore not fit to be reasoned with) when in fact you're just calmly pointing out that there's no need for that kind of tone towards someone that thinks differently than you.
I'm more than comfortable with calling someone out on their behaviour but more often than not those kinds of people aren't worth the effort as they'll never let it go because want to get the last post, and also because they probably love the attention.
So most of the time, if I think this is how it's going to go down, it's just not worth it. As I think it's pretty easy to tell beforehand (no matter how many preemptively prepared disclaimers you put in) when there's an agenda and people don't want to be challenged, but if they are they can't help but argue to get the last post. Again, this is commentary on the posting on the internet in general... Look at that I'm even doing some preemptive disclaimers now! Haha