I got it to happen again finally and this time, I got a screenshot of it. It's very rare, but it's still happening:
Notice the white area with "Error" in it. I looked at the HTML source of the page, searching for that string, "error", but nothing seems to hint toward an image (from the "img" tag) so I can't tell what the source is supposed to be.
For reference, I'm using Firefox 7.0.1 under Windows XP Pro SP3. I have zero add-ons and extensions beyond nothing more than Flash.