After Abraham tries on Gabe's glasses, he takes them off but we never see Gabe get them back. Gabe is wearing them in the next scene.
When the Tethered kill the Tyler family, the shot from outside the house shows Kitty and Josh falling down very close to each other. In the next shot of Kitty, she is now crawling toward Josh who is now several feet away from Kitty.
The doppelganger girl attacks the windshield of the SUV with her scissors and cracks it but in the morning when get to the beach the cracks have gone.
In one scene Adelaide handcuffs herself to the table, but in later scenes she has the handcuffs on both wrists.
When Gabe was talking to Addy in their bedroom, he was in his plaid shorts. Then Jason got to tell them about the outsiders, they moved right away to the front door. Gabe was already in his trousers when he got out.
No one in the tunnels would have survived as long as they did with rabbit as their only food source. The human body has a minimum fat intake requirement per day, and rabbit meat is so lean it's almost completely devoid of any fat. Eating a diet of solely rabbit leads to a phenomenon known as 'rabbit starvation'.
In the song credits at the end of the film, "Les Fleurs" by Minnie Riperton has her last name spelled incorrectly as "Ripperton".
No combustion engine will ever start, by simply slamming on it.
The tethered are depicted as spiteful and vengeful, so then why aren't they hunting down the government scientists who created and abandoned them? They're attacking innocent people whose only crime is not knowing their clones existed. But the Tethered are not very bright, and Red, who set up their mission, has an element of religious fanaticism. It is not based on rationality.
Adelaide struggled to reach the fireplace poker (when handcuffed to the table) despite the table being light enough to push/drag closer to the fireplace.
When young Adelaide entered the house of mirrors on the beach, it suddenly began raining. However, the wooden deck at the entrance was already entirely drenched, presumably from previous takes.
When the family is seen walking on the sandy beach from above, the sun casts long shadows of them, but on the next series of shots, the shadows of other people on the beach are much shorter, revealing that the first shot was taken at sunset.
At the beginning of the movie (when it is 1986) there is a commercial for the "Santa Cruz Beach Boardwalk" that show a shot of a "No Fear: Dangerous Sports" pinball machine. This pinball machine was not released until May of 1995.
At 1:05:18, there's a crew member outside of the window that Josh looks out of.
At about 1 hour : 33 minutes: 40 seconds , a boom microphone is seen hanging above Addy while she descends into the underground chamber.
The Tethered live on the meat of rabbits provided for them. Since the Tethered are too stupid to understand cooking procedures, they would eat them raw, resulting in mass food poisoning. Furthermore, what do the rabbits live on? There are never any botanical or hydroponic gardens depicted in Tetheredland. And rabbits are notorious for eating more than nature can provide.
Tethered presumably can die of old age and other natural causes, or of blunt force accidents caused by inconvenient movements of their upper world analogs. Who removes the corpses and prevents disease from rapidly spreading through Tetheredland?
How do the Tethered have near-exact copies of their human counterparts' clothing?
Why didn't the government euthanize the clones once the project was canned 30 years prior? And why did they clone every American citizen instead of just testing the waters with a few clones instead? Leaving them abandoned to fend for themselves in underground tunnels leaves the risk of civilians discovering them or the clones escaping, which is exactly what happens.
How was Red able to contact every single tethered across America to plan an attack on the country? The tunnels they live in do not seem to have any forms of long distance communication.