A very common misspelling of mince pies, often done accidentally when you miss the "n" key because you are in a rush.
Can also refer to a small pie, similar to a mince pie, but instead filled with mouse droppings, served during christmas time to people who give you shite presents.
Tony: Hey, I'm off to get another mice pie, brb.
Dave: Mice pie? WTF?
Tony: SHIT! I mean mince pie! My bad.
Tony: Hey, go give this mice pie to Uncle Freddy
Dave: Yeah! that would totally get him back for buying me another fucking lynx box set this Christmas!