You remember the Leaf? It's like that, only the other way around.
Behind the Scenes
It's rumored that the actor playing Pete was let go after only three episodes due to confusion arising on the set due to directors referring to him by his character's name. Capaldino, being supreme commander of the program, ordered that either they change the character's name or fire him. As two episodes were already in the can, Moffat saw no alternative but to fire Pete and hastily write him out of Thin Ice.
In actual fact, the reasons for Pete's erasure were more sinister than we could have ever imagined (pic related).