I can assure you it's not your imagination. It's a known quirk/minor inconvenience that has been around for several years. I don't know why it sometimes flags the HTML as wrong. Sometimes it happens, sometimes it doesn't. I post a lot, so I usually encounter it every day, but not always. Whenever that pops up for me, if I know nothing's wrong with it, I just click to submit it again. I don't have to use the Preview method. You should be able to just submit it again--at least that works for me. I've never had it not submit on the second attempt. If it continues to show you that "error" on multiple attempts and you can't get it to re-submit at all without using Preview, then that's different.
