The Mirror-Sigil Sergeant is only supposed to replicate itself once, at the beginning of upkeep, but instead it replicates itself many times until it fills the screen and the game crashes with a "No room to add a card." error.

Edit: I'm sorry, let me correct myself - the game doesn't crash. You get a bazillion Sergeants, but you can click on the error box and keep playing. I guess I was too hasty writing that.