Sometimes readers put URLs in their comments. That's good. Sometimes they format them as HTML URLs with short text to click on. That's even better. If we're just in the 'good' zone, then funny things can happen while a comment with a long URL is one of the five most recent comments showing in the right margin—at least in most browsers.
Apparently, if you read this blog with Firefox, it handles comments with long URLs fine. But I gather that if you use pretty much anything else, it doesn't. So when people put a long URL in plaintext into their comments, and it shows up in the right margin as one of the five most recent, it shoves that column left, making it look as if the middle column got too big.
I've fixed the problem for today by editing the comment that was causing the problem, but if there's a more general fix, I don't know what it is, since MT's parser isn't built to deal with this.
Thank you to the people who were kind enough to warn me about the problem. I'll keep an eye out for it.