I just tried this thread using IE9 on Windows 7 32-bit and I also sometimes gets errors.
So far they all was for Google ads.
The failed ones had this in them googleads.g.doubleclick.net.
I only had the top banner add and the upper ad on the right side fail for me.
Here's one of the failed ones.
Edit: I just realized that having flash for Firefox does not mean I have it for IE9; downloading it now.
Edit2: I realized those spots are blank for me using Firefox; so maybe I do not have flash for it either.
The error messages do look bad under IE9; while the white spots look normal under Firefox.