Uh-oh. Commenters on Epic’s forums have recently been reporting that a glitch in Gears of War 3 is causing some consoles to freeze up. It seems that if you try to play Horde mode in splitscreen with two profiles, the bug attacks and permanently screws up one of the profiles, blocking that person from entering ANY other game modes in Gears of War 3.
There is reason to hope however. Epic’s Rykard has posted a questionnaire that attempts to find out what exactly is going on and update the game to fix it. Until then, we advise you NOT to play Horde mode in splitscreen.
Source: EPIC Games