Comment 5 for bug 769650

You've exceeded your hardware's maximum texture size (2048 x 2048). See LP: #555641 which this is basically a dupe of. Sounds like X switched to software rendering rather than crashing or something equally antisocial, so that's good.

The real bug here is that gnome-display-properties apparently doesn't doublecheck the max texture size when compiz is active. It needs to tell the user the consequences of setting the root framebuffer size to be bigger than the max texture size when they try to apply it.