I'm reporting this as a bug, but it actually looked pretty kewl and maybe should be a feature...
I have not been able to recreate it, but can say it happened when I clicked on the main menu button to look at the credits (thank you!!) when the game was still loading.
the menus all game up but they were faded out and overlaid against my live game. probably should be the game that is faded and not the menues, but still... it looked kewl.