Plot hole: When Dana realizes that the puppeteers made them choose their own deaths (while standing in front of Fornicus in the elevator) she does so by watching the puzzle ball he holds. But she never saw the puzzle ball back in the cellar. Only Curt handled it briefly and is shown approaching her without it when she starts reading the diary.
The Cabin in the Woods (2012)
Ending / spoiler
Directed by: Drew Goddard
Starring: Chris Hemsworth, Anna Hutchison, Kristen Connolly, Fran Kranz
By the end, only Dana and Marty are left. They make their way down to the underground control room and let loose all the horror movie monsters meant as potential killers for them. The creatures kill all the workers. The Director of the whole operation tells Dana and Marty that the whole thing was a staged ritual to sacrifice people to the gods that ruled the Earth years ago and now reside underground. If the gods don't recieve a proper sacrifice, they rise and destroy the world. Dana considers killing Marty to save the world, but she is attacked by a werewolf. Marty fights the Director, who gets killed by a zombie. Bloody and battered, Dana and Marty share a joint as the gods rise to destroy the world.
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More mistakes in The Cabin in the Woods
Dana: Thanks for... Being decent.
Holden: Least I could do, since Curt and Jules have sold you to me for marriage.
Trivia: Actual "Cannabis Experts" were brought in to consult with actor Fran Kranz, so he could properly portray a convincing stoner in the film.
Question: Is there a reason, that a facility full of monsters would ever, under any circumstances have a System Purge button? Is it just to make satire of the fact that the antagonist is usually stupid?
Answer: As you said, it is a satire of the common Deus Ex Machina trope you find in such films.