Revealing mistake: When Mark goes to treat his cut, he leaves the lounge with a couple of tissues held to his head. He gets some supplies and puts a supposedly fresh piece of gauze to his forehead - however the gauze already has some "blood" on it - a feature the camera obviously wasn't supposed to catch. When Jeanie comes over to check it out, he pulls the gauze away from himself and - voila - there's the bloodstain.
Revealing mistake: Near the beginning of the show, Mark's father puts a bulb into an old slide projector. Then there is a small explosion with smoke and flames - watched carefully, we can see are actually coming from the other side of the projector - the flames can be seen shooting parallel and close to the floor, which wouldn't have happened if the explosion actually came from the projector.
Revealing mistake: Gallant is watching over some duck eggs that are about to hatch. At the end of the episode, two of the ducklings have hatched and Gallant is showing them to Carter and Susan. An unhatched egg falls out of Gallant's bundle and smashes on the street.The smashed egg has a large yellow yolk and clear fluid, like an egg in a grocery store. But an egg that close to hatching would have almost no yolk remaining and an almost fully developed duckling inside.
Revealing mistake: Sam's mother is standing at an open window, smoking a cigarette. The outside of the window is covered with some sort of frosted glass or ice or something (making it pointless to have it open anyway). Two shots later, we see her close the window from a different angle. This time, the window is clear and you can see the brick building next door. (00:19:36)
Revealing mistake: After Mark gives his father the keys and tells him to park the van, in a following shot when Elizabeth joins the gurney rush with Carter, as she's walking backwards there's a distinctive large bulge at the back of her waist from the mic transmitter. (00:10:55)