Oh yes, now I know what you mean. The Captcha, one of those "I am not a robot" tests. Strange that we only get it once in a while, as you said. You probably remember when there used to be a "type this word" sort of captcha test before posting, and it was every time. It was less elaborate, but also annoying because sometimes it was impossible to read the distorted word you were supposed to type. It's an unfortunate outcome of spambots, that Bravenet feels the need to do this to prevent junk auto-posts, and we still get a spam post every now and then (which I remove when I see them). Before that, we didn't have any anti-spam protection on the forum at all, and unfortunately the junk posts got really intrusive and I had to turn on the security feature. Now, I don't think Bravenet even gives us a choice, because the robots are relentless. I share your annoyance about it, but I'm afraid there's no perfect compromise between convenience and security, and it's out of our hands at this point.
It's a little like Murphy's Law, because it usually happens when I've finished typing something longish. Not ten paragraphs or anything like that, but longish. And of course that's when it decides to happen, so I'm unable to post it.