Comment 3 for bug 784209

I don't think it's a unity-2d specific error because I get it with compiz as well and on different systems.

This error occurs when the background is set to a fixed color (not gradient) rather than a picture but I haven't been able to figure out what causes this. The 'trailing effect' is visible even before login, when the gdm window is resized when the user selects an account.