Revealing mistake: When Francis emerges from the cave, he stands in the foreground, with the distant mountains and sun in the background. The sun creates a lens flare, which is an internal reflection within the camera lens. Yet Francis is in front of the flare. This shows that the actor was composited with the background via a traveling matte process.
Revealing mistake: In the carousel scene, when the contestants are starting to float into the air, you can see the cables attached to the contestants that are lifting them up. (00:08:58)
Revealing mistake: When the camera is in a close up on Box's face, you can see the actor's mouth inside the mask.
Revealing mistake: When Logan shoots at the first runner, his two shots hits the ground in front of the runner but you can see the setup on the ground that explodes and creates sparks. (00:11:36)
Revealing mistake: When the dome city starts to blow up, people are running around in a panic. One man in a red outfit jumps off a balcony to the next floor and you can see a modern (1970s) lit exit sign over a doorway behind him. Many of the interior shots for the dome city were filmed in a large shopping mall; that sign must have been missed when they were trying to make it look like the year 2274.
Revealing mistake: At the surgeon, the laser cuts the stretcher (a superimposed red light), but when the lasers go berserk they don't cut anything, revealing they're just lights.
Revealing mistake: When the film starts Logan and Francis chase a runner who stands by a column, the squibs used to allow for the sparks and FX are visible.
Revealing mistake: In the opening scene at the nursery, a flesh colored clip is visible on the baby's hand. It is holding the white crystal to the center of his palm. (00:02:48)
Answer: In popular American lore, the Sandman is responsible for putting sand over your eyes to make you go to sleep. In this context the "sleep" is a euphemism for death, and the enforcers are responsible for making sure that everyone dies on time.
Phoenix