Dupli-Kate's twin brother Multi-Paul has a cameo.
Mark mentions a comic to Amber about an underwater welder who sees ghosts. This is based on "Underwater Welder," a real comic by writer/artist Jeff Lemire.
Damien emulates Rorschach's speech pattern, from 'Watchmen". it's particularly evident in this episode, when Damien makes the same "Hurm" noise that Rorschach does in the comics and the movie.