Gears of War 3 players are flooding the Epic forums with complaints of freezing issues, which occur when players attempt to load a game mode.Specifically, it seems to be limited to splitscreen Horde play with two profiles -- when the glitch hits, it permanently alters one of the profiles, making it impossible to enter any game mode. So ... that's not a great bug.
Epic is aware of it; in fact, Epic rep Rykard posted a questionnaire attempting to nail down the exact issue. We're checking in with Epic for an ETA on the fix. Until then ... try not to play Horde mode in splitscreen.
Update: Epic Games tells us, "Getting this resolved is a priority for Epic. We are collecting information internally, sourcing as much as possible from the community and exploring potential fixes. We do not have any recommendations to share with players just yet."