Re Watch Bonus: The talisman Sitterson kisses on the first death has the same pattern as the floor in the room with the five sculptures at the end.
Similarly, rewatch the scene with the talismans in the basement and try to figure out which one belongs to which monster in the containment rooms.
Star-Making Role: Chris Hemsworth. According to Joss Whedon and Drew Goddard the dailies from the scene where Curt reveals Jules has been killed were so well-received by the studios that it actually locked him in for lead in the remake of Red Dawn (2012) and got him a callback for Thor which he would come to star in.
Steven Ulysses Perhero: The greensperson (crew member in charge of acquiring plants and trees for a film set) on the film is named "Glenn Foerster". Drew Goddard and Joss Whedon mention on the commentary how thrilled they were at his name.
In both this film and Paul, Sigourney Weaver plays the head of an evil organization hiding an otherworldly secret from humanity; she appears only by voice until the very end of the movie, when she presents herself and tries to kill the protagonists, only to be killed herself. (then again, both serve as a Casting Gag, inverting her most famous role of someone who gets screwed by a Mega Corp.'s questionable decisions)
What Could Have Been: The various Left 4 Dead characters are present because there was supposed to be special L4D downloadable content based on the film. The deal fell through, but the characters didn't seem out of place, so it was easier to keep them in.