Corrected entry: Once the characters have all arrived at the cabin in the woods, there is a scene during a game of "Truth or Dare" where they all enter the basement. During that scene, Jesse Williams' character "Holden" magically acquires glasses that he was not wearing at any point prior. From that point on, throughout the remainder of the movie, his glasses disappear and re-appear multiple times. Most notably, in the scene where three of them attempt to flee through the stone tunnel in the RV.
The Cabin in the Woods (2012)
1 corrected entry
Directed by: Drew Goddard
Starring: Chris Hemsworth, Anna Hutchison, Kristen Connolly, Fran Kranz
Continuity mistake: Just before Steve Hadley is killed by the Merman, his tie is wrapped around his neck. When the Merman attacks him, his tie is worn properly. (01:19:40)
Dana: Thanks for... Being decent.
Holden: Least I could do, since Curt and Jules have sold you to me for marriage.
Trivia: One of the creatures glimpsed jumping out of the lift is one of the small monsters which attack people in the subway in "Cloverfield". The director of this movie, Drew Goddard, wrote Cloverfield.
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.
Correction: Just because someone wears glasses that doesn't mean that they need to wear them all the time. He may only need them for reading, and he has plenty of opportunities throughout the film to take them off and put them on.
THGhost