According to the Marvel Zombies story, Magneto was one of the last remaining non-converted who eventually became overwhelmed by a mob of zombies. When the Hulk showed up, he tore Magento apart like chicken-fried-chicken and swallowed his leg. When he changed back to Banner, the leg pushed a hole through his gut, and stayed wedged there. He was lucky the leg got stuck there and not somewhere else that might've been obstructing to his walk. But he's still got the rest of Magneto to devour - starting with the head.
Hope this brings lots of laughs, thanks for checking it out!