Rendering issue: Webplayer, Windows 8 only

Hello -

We have a game for Unity WebPlayers, which is showing a rendering bug - but in Windows 8 browsers only. (It's happened in old and new versions of FF, and the latest Chrome.) The game looks fine on MacOS, Win 7, etc., but on Win 8 there is a strange purple shape that appears on both sides of the screen.

Attached are 'good' and 'bad' images. Note the addition of a strange purple overlay in the bad image. Also note that these are both screencaps from our live, production server - the exact same codebase and .unity3d file. We've tried on several machines, and so far it looks like the single predicting factor of whether it happens or not is Windows 8.

Good:

Bad:

We're not even quite sure how to proceed, since there are no objects in the scene that we've put there to create that purple overlay (in the editor or programmatically). Could it be related to lighting? Some other different implementation or limitation that happens only on the Win 8 web player? We could really use help with this one.

Thanks! Aaron