Zombie player walks to his position in the middle of play when you pull a double switch, and you'll only see it if the path the player takes to his position can be seen from the pitching view. Obviously, swapping the catcher delivers the most hilarious results, seen here.
A double switch is a semi-exotic managerial move designed to replace two players, one of them often the pitcher, and swap their positions in the batting order to get the better hitter up in the next half inning. It's not so uncommon that this is at all acceptable, but its relative lack of use and the fact a catcher needed to be involved is probably why this hasn't been noticed until now.
This glitch will not be patched until May at the earliest. That's likely because of MLB 2K12's Perfect Game contest, which gets underway on April 4. Last year's game was not patched until mid-May, as well.
MLB 2K12 Video - Zombie Player Glitch, When Performing a Double Switch [Operation Sports]