Plenty of times. Sometimes I even submit them and delete them a minute later.
I felt like I had something interesting to say, but when I actually articulated it, it didn't add much to the conversation. If you noticed PG's essay that he wrote with Etherpad, he had entire paragraphs in his draft that didn't make it to the final version.
Even more than that, sometimes while writing a comment I'll realize my position is flawed. I generally try and make sure my argument is sound before I submit, because I know you can't get by on hand-waving here. Often in my search for data to support my case, or even when I just try to articulate it properly, I find that I'm actually arguing the wrong side of something.
Being forced to re-examine my own positions is one of the things I enjoy most about a discussion, whether it's caused by something someone else says or something I almost said, then thought better of.
(Also, most things I've submitted within the last 10 minutes are being furiously edited. Not sure why, but I don't notice a lot of things until I read my own post in the context of the others.)
Yeah I found that helped me be a bit more mature on online forums. In high school I would use pseudo-anonymous screen names, which perhaps allowed me to say things I wouldn't really want attributed to my real name, even though it was still trivial to link it to my real name. Now I just use my initials + last name, which is a good reminder that you're definitely not really anonymous on the internet.
I saw the question and wanted to answer something along these lines. So I opened the link and saw that I have nothing to add other than "+1". Now I'm thinking if there's a point in submitting this comment. Oh well, once in a while I'll submit what I decided not to submit. Do I make sense at all?
I felt like I had something interesting to say, but when I actually articulated it, it didn't add much to the conversation. If you noticed PG's essay that he wrote with Etherpad, he had entire paragraphs in his draft that didn't make it to the final version.