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.
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. While it is possible that David could have been referring to a second, off-screen, break-up conversation with Kathryn the day she disappeared, 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.
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?"
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).
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.
Revealing mistake: When Sheriff Graham pulls in front of Emma in a police car, the big sign on the storefront behind Emma says "Storybrooke Coffee Co." However, as Graham gets out of the car, you can see a circular sign on the side of the building, which says "Steveston Coffee Co"; Steveston is the Canadian town which doubles as the backdrop for the town of Storybrooke on Once Upon a Time.
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.
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.
Revealing mistake: When Emma walks Henry to school, as Emma says "Because I'm the daughter of Snow White and Prince Charming?", a reflection of the clock tower building is visible in the window behind her, only there's no clock (the top two layers of the clock tower, including the clock, are visual effects).
Continuity mistake: When Mary Margaret gets thrown by the Lily dragon into a rock, her blood smear on her head is large and messy. They cut away and cut back to her quickly and it is nice and neat with two blood drop lines slowly forming away from the cut.
Revealing mistake: When Snow White and Prince Charming hurriedly make their way through the woods in the beginning of the episode, in the shot where Snow White jumps over a log, you can see the face of Ginnifer Goodwin's and Josh Dallas' stunt doubles. Josh Dallas' stunt double can also be seen as Snow and Charming are running thought the woods in the previous shot.
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.