The rather literally named "
Fugue and Dies Iræ" from
Sam & Max Season Two (specifically, episode 5) starts off with a real fugue and then busts out Dies Irae at the climax.
This tune is pretty much Jared Emerson-Johnson's crowning moment of genius, by the way. It's insane that it only plays in this small hub room you only spend like thirty seconds at a time in.