Factual error: As the Royal Navy has its own police (including detectives), there would be no need for a civilian detective to be airlifted onto a submarine, especially to investigate a sudden death that was not originally believed to be a murder.
Continuity mistake: When Sam is in the bathroom holding up two sample paint chips, in the first shot she's holding up the yellow in her left hand and the blue in her right hand, but in the next shot they've switched and are in the opposite hands, just as Jay arrives with the coffee.
Other mistake: The premise of the first episode is flawed; "a single choice" should make the difference, the choice being identified as Ms. Carter not leaving the room to get in the observation booth. But in the normal timeline the booth is exactly where all the other characters were. In this reality everyone else is downstairs, with the German spy that inexplicably sets the bomb early. If anything, it's his bizarre, suicidal choice that matters.
Factual error: At the end of episode 6, we see Alain crossing the German French border in a European registered right-hand drive Citroen CX. Then at the very beginning of Episode 7, they check into a hotel and the Mercedes parked in the forecourt was also right-hand drive. (00:01:00 - 00:01:30)
Continuity mistake: One of the 4400 now has an apartment and shows off her new darkroom to a friend. It has a sliding door at the entrance. In a later scene she is in the darkroom and her friend enters. But this time the door is a regular one that swings. In truth, neither of these are typical of a darkroom, as darkrooms have a light lock entry. Either double doors or a U-turn entry to prevent light from getting in.
Continuity mistake: When Mark got into his cell, the top bunk had a vinyl-like covered mattress (or folded gym mat) that had a lengthwise seam along the middle of the side. He was not fixing his bunk bed while he was talking to cellmate Bernard, but when he was removing items from his bag on top of the bunk bed, it was now covered with a blue blanket and a sheet folded down by the pillow could be seen. A little later, the blanket was also covering the foot of the bed. (00:13:15 - 00:15:03)
Plot hole: When the bandits attack the pioneer's wagons they are chased on horseback by the Captain, Jack and others for quite a distance until Ennis is killed in the confrontation with one of the bandits. In the following scenes Elsa is mourning over his body but her mother and others from the wagon train stroll up and the wagons are in the background-all this after a fairly long chase.
Revealing mistake: When the big battle starts, Boba and Mando appear using their jet packs. As Boba lands with one foot forwards, sand is thrown up into the air, but the sand is a special effect coming from about a metre in front of Boba, not from the foot itself. The result is very unconvincing.