If has been confirmed that there's an issue with AMD drivers in crossfire mode when using GPU:3D in the following configuration :

• Error Checking mode ON
• FullScreen mode ON
• Shader Complexity 8 only (others work fine)
• AMD Crossfire configuration only

You'll get false error reported.

I do think this is a driver issue, as this configuration works fine in single cards, Nvidia, AMD, Intel, or in SLI mode (with Nvidia cards, confirmed on 2xGTX285 ).

The workaround is just to use Shader Complexity 7 (or less) to get around this problem on AMD Crossfire configuration.

I'm working on solving this issue. It might take some time, as i cannot reproduce it yet personally.