Ugh...
Well, my very first Opera post here revealed a startling discovery. WordPress rich text editor hates Opera with a passion. It's a known bug, so maybe they'll actually get around to fixing it. Seems to be some funky word-wrapping error. When you type, it usually wraps at the edge of the field, buuut... In Opera, sometimes it doesn't! And when it doesn't wrap properly, then it somehow translates the error directly to the post and you've got text spilling all over your site... And that shows up in other browsers, too.
So maybe I'll just not use the rich text editor. Somehow, I like not using it. It gives me more precise control over how my posts will look... I can see all the HTML tags being applied and all that kewl stuffz. But blarg, I say. Why can't things just work the way they're supposed to. On the bright side, I guess Opera is the one that's standards compliant, so it's just WordPress being dumb.