Visible crew/equipment: When the horse carriage is ambushed, just as Princess Abigail can be heard screaming "Please, help me", the camera pans downward and a horse wrangler wearing a blue baseball cap can be seen in the lower left hand corner, hiding behind one of the horses.
Revealing mistake: After the scene with Emma and Henry at the playground, there is a wide shot of Storybrooke Main Street. As the camera pans down, in the upper right hand corner, you can see the clock tower, but without the top two stories (the top two layers of the clock tower, including the clock, are CGI).
Character mistake: In the evening, Emma states that Kathryn Nolan has been missing for 24 hours, meaning that this episode takes place the day after Kathryn disappeared. When Emma investigates Kathryn's car, David tells Emma that he last spoke to Kathryn "yesterday afternoon" (and stresses that he is telling the truth), and that he hasn't spoken to her since he ended their relationship. However, "What Happened to Frederick" establishes that David broke up with Kathryn the day before she tried to leave town and disappeared, because when Kathryn apologized to Regina, Regina referred to the events surrounding the break-up as "what happened yesterday." Emma checks Kathryn's phone records and finds out that Kathryn talked to David on the phone the day she disappeared, but David could not have referred to that conversation, since "Red-Handed" and "Heart of Darkness" both establish that he doesn't remember the phone call because of a memory blackout. Making matters even more complicated is that when Sidney shows up with the phone records, Emma specifically says to him that David told her that he DIDN'T speak to Kathryn that day, which, in fact, he said he did.
Revealing mistake: In the first shot of the scene where Emma and the others arrive in Storybrooke in their cars, on the left side of the screen, you can see a sign behind a branch. It says "Steveston Museum and Visitor Centre." Steveston is the name of Canadian town which doubles as the town of Storybrooke on Once Upon a Time.
Deliberate mistake: Elsa's dress in Season Four is different from the dress she was wearing in the Season Three finale. The new dress is a different shade of blue, and has a completely different design, even though the present day events of Season Four take place mere moments after the end of Season Three.
Continuity mistake: Emma has a flower tattoo on her left wrist. Throughout the series, there are subtle changes in its design, and its position on her arm. For example, when Emma is sitting at the table with Mary Margaret in "The Thing You Love Most", the tattoo has six petals, and is located about an inch below her hand. When she's on the phone with Ruby in "Red-Handed", the tattoo is much bigger, and the petals are arranged differently; it has also moved further up her arm, and is almost touching her hand. When she's arrested in "Tallahassee", there are five petals instead of six, the petals have a rounder shape (in "Red-Handed", they were rather pointy), and the tip of one of the petals is touching her hand. When she pulls her hair behind her ears at the tavern in "Snow Drifts", said petal is thinner, and is no longer touching her hand. When she touches Hook's face in the diner in "Birth", the tattoo has moved about a centimeter down her wrist, and the petals have a brownish color, but in previous episodes, the petals have no color. When she touches Hook's face in the Middlemist Field in the next scene, the color is gone.
Continuity mistake: When Regina is standing in front of Rumpelstiltskin's mirror at the end of the episode, as he asks, "What are you gonna do now, then?", the cloth that was used to cover the mirror is lying on the floor. When he transforms her back to her normal form, it is gone.
Continuity mistake: When Killian Jones, a.k.a. Captain Hook, is talking to Emma on the street, he points to one of his rings and reveals that it belonged to a man who insulted him by calling him "One Hand Jones." According to Hook, he killed the man in front of his wife, and took his ring as a trophy. However, as seen in 2-04 "The Crocodile", Hook was already wearing this ring before he lost his hand to Rumplestiltskin. This is best seen while he and Milah are drinking and gambling at the tavern: In the shot of the dice landing on the table, you can see Hook's hand resting on the table, wearing the same ring.
Continuity mistake: In the beginning of the episode, as Regina stops below the clock tower and gazes at the clock, there is no sign on the blue building in the background, next to Storybrooke Country Bread. When she notices Emma's car, there is a sign on the building.
Other mistake: In the October 2011 flashback, when Mary Margaret gives Henry the storybook for the first time, he opens it to show Pinocchio and Geppetto in an illustration. However, according to 1-20 "The Stranger", Pinocchio's story was not added to the storybook until later after August came to Storybrooke, several months later. This is confirmed in 4-13 "Unforgiven", where Regina and Henry look at the exact same picture, and notice that the paper is different than the rest of the book, because this particular story was added by August.
Continuity mistake: The flashback scene where the Snow Queen takes Emma's video camera away from Kevin differs from what is seen on the video recording of that moment in "Breaking Glass" and "Family Business." For example, in the recording, Ingrid says, "We respect property in this house, Emma." In the actual scene, she says, "We respect property in this household, don't we, Kevin?"
Continuity mistake: When Emma arrives in town, two Storybrooke businesses, Standard Clocks and its neighbour, Modern Fashions, can be seen in a shot though a window. In the shot from above where Emma's car slows down, the name on Standard Clocks is Kaido (a real-life business in Steveston, the Canadian town which doubles as the town of Storybrooke), and there is now a white sign on Modern Fashions, which wasn't there in the first shot.
Continuity mistake: The exterior shots of the buildings located across the street from Granny's Diner do not match the view seen through the front door, from inside the diner. The most obvious example is the building next to Storybrooke Country Bread, on the opposite side of the street. In the exterior shots, on the left side of the bakery, there is a blue wooden building (this can be seen, for example, when Emma is walking Henry to school in "The Thing You Love Most"), with a huge gap of approximately ten meters between the two buildings. In the interior shots where the front door of Granny's Diner is open, the view is drastically different looking: For example, when Regina enters the diner to confront Mary Margaret in "The Shepherd", when Emma brings Henry to the diner in "Welcome to Storybrooke", when Zelena enters the diner in "It's Not Easy Being Green" and when Emma comes into Granny's Diner to ask Hook out in "The Apprentice", the characters open the front door, and now there is a completely different building next to Storybrooke Country Bread. The new building seen through the doorway is made of bricks and has a completely different color, and there is no longer any gap between the two buildings; they are touching each other. Also, in some episodes, like "The Shepherd" and "7:15 AM", in the scenes inside the diner, you can see a grey wall on top of Storybrooke Country Bread. In exterior shots, there is no such wall.