Other mistake: Katie cuts herself with a pair of scissors but when the girl hits her there's no mark there.

Us (2019)
1 other mistake - chronological order
Directed by: Jordan Peele
Starring: Elisabeth Moss, Lupita Nyong'o, Yahya Abdul-Mateen II, Anna Diop
Continuity mistake: Red is barefoot for a long period in the movie, but when they run to the boat she's wearing barely-visible slippers, presumably to protect her feet while running outside.
Adelaide Wilson: They look exactly like us. They think like us. They know where we are. We need to move and keep moving. They won't stop until they kill us... or we kill them.
Trivia: Spoilers: Among the video-tapes seen next to the TV after the opening credits are "C.H.U.D." and "The Goonies." This is quite relevant to the plot, in which it is revealed that the doppelgangers come from beneath the ground in a series of tunnels that used to be run by the government. "C.H.U.D." is about mutant creatures that live underground and have began to rise up and attack, like the doppelgangers, and "Goonies" takes place largely in underground tunnels and caverns, much like the film's climax.
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Answer: The project was abandoned, but they obviously kept the tethered ones alive. This may have been out of some sense of compassion, or because there was a possibility they would be useful in the future.