Revealing mistake: As Hook is chasing King Arthur through the woods, there is a shot of Arthur running, where you can see that the one running is not Liam Garrigan, but his stunt double. In the next shot, Hook is hot on Arthur's heels and you can see that it is Colin O'Donughue's stunt double who is chasing him.
Plot hole: To wake Mr. Gold/Rumplestiltskin from his coma, Emma needs an object that touched him before he became the Dark One. Hook shows her a sword, telling her how he held it to Rumplestiltskin's head and taunted him. However, the sword that Hook shows to Emma, is not the sword that he threatened Rumplestiltskin with in "The Crocodile." The sword that he used to threaten him had a unique, golden handle. When Hook and Rumplestiltskin met again a few years later, Rumplestiltskin took the same sword from Hook and dueled him with it. When Rumplestiltskin vanished into thin air after cutting off Hook's hand, he took the sword with him. The sword that Hook shows to Emma, which Emma uses to wake Mr. Gold, has a black handle with a completely different design, and is the sword Hook carried after the other one was taken from him; he can be seen using it in episodes like "Queen of Hearts", "Lost Girl" and "The Jolly Roger." This means that the sword Emma uses to wake Mr. Gold from his coma never actually touched him at all; however, the spell still works.
Revealing mistake: When Hook tries to rip Zelena's heart out, as a blast of magic hurls him into the wall, you can see that the one who actually hits the wall is Colin O'Donughue's stunt double.
Other mistake: When Belle and Gold run from Merida in Gold's shop, she shoots an arrow through the door. We see the end of the arrow (with the white feathers) sticking out of the other side of the door. When when Merida pushes the doors open, the end of the arrow is missing, even though the arrowhead end is still sticking out.
Factual error: The episode opens with Emma going to the State Theater in Mpls. in 1989. The theater was closed for renovation at that time, and hadn't shown a movie since 1975. It was the Jesus People church all through the 80's.
Continuity mistake: When we see Emma giving Merida back her heart, shoving her away in the same motion, a couple of shots later the camera turns on Emma and we are shown her still doing the same movement, although several seconds have passed. (00:33:40)
Revealing mistake: During the car chase scene, when Lily is driving Emma's car, there are several wide shots of the car driving down the road and you can see that the one driving is not Agnes Bruckner, but her stunt double. Lana Parilla's stunt double can also be seen in several wide shots when Emma and Regina catch up with Lily and try to overtake her. (00:30:55)
Welcome to Storybrooke - S2-E17
Revealing mistake: After Regina burns the curse that will make Henry think he loves her, when Henry walks over to Emma and the others a few shots later, Regina has the curse scroll in her hand again. (00:36:40)
Revealing mistake: As Moe French steps out of his florist's van, he is greeted by Mr. Gold and his lackey. As. Gold says, "Well, this is just perfect", in the background, you can see the clock tower building, but without the clock tower (the actual clock tower is CGI). (00:03:50)
Quiet Minds - S3-E15
Continuity mistake: When the Vault of the Dark One opens, there is a wide shot where Neal turns around, and you can see that the area around the circular vault is covered in snow. In the next shot, a large area around the vault is free from snow. The amount of snow around the vault keeps changing during the rest of the scene. (00:26:45)
Revealing mistake: When Hook is down in the pit, trying to escape the undead Maleficent, after he says, "Maleficent. Love you in earth tones", there is a wide shot of Hook and Maleficent where you can see the face of Colin O'Donoghue's stunt double. The stunt double's face can also be seen in the shot where Hook stands up after climbing on top of the rock. (00:22:20)
Snow Drifts - S3-E21
Continuity mistake: At the docks, when past Hook lifts Emma up, as he says, "Oh, no need", Emma is on her feet, facing him as he begins to pick her up. In the next shot, Emma is facing a different direction, her back leaning against his shoulder/arm as he picks her up and starts carrying her. (00:32:10)
Continuity mistake: When Emma arrives at the restaurant, as the elevator opens, a waiter walks past. In the next shot, the waiter walks past the elevator again. (00:04:55)
Continuity mistake: When Emma meets up with Ryan at the restaurant, as he says, "Well, it is the Internet", Ryan is positioned next to Emma and moves to walk around her. In the next shot, he is already standing behind her as he pulls out Emma's chair. (00:05:15)
The Return - S1-E19
Continuity mistake: When August drives off on his motorcycle, in the close-up of his foot as he walks, the pavement he steps on has concrete paver blocks. In the next shot, where he sits on the motorcycle, the motorcycle is parked in front of the Storybrooke Pet Shelter, and the pavement has changed to asphalt concrete. In the next shot, the vehicle, seen from a different angle, is parked in front of the business located on the left side of the pet shelter; easily seen from the pavement, which is now paver blocks again. (00:16:45)
Continuity mistake: When Emma is driving through downtown Storybrooke, in some shots, the real-life businesses in Steveston Village (the Canadian town which doubles as the town of Storybrooke on Once Upon a Time) have not yet been transformed into Storybrooke businesses. For example, a sign on a building on the left side of the road (in the first shot) says Pacific Net and Twine, and on the other side of the street (in the shot where Henry is heard saying, "Forty-four, not-telling-you street"), the name on the local bakery, which is Romania Country Bread, and is sitting next to two businesses called Kaido and Splash. In "The Thing You Love Most", the buildings have been transformed into the Storybrooke businesses seen throughout the rest of the series: For example, Pacific Net and Twine has become Atlantic Twine and Net, the bakery has been renamed Storybrooke Country Bread, and Kaido and Splash have become Standard Clocks and Neighbor's, respectively. Also, in "The Heart is a Lonely Hunter", when Graham first meets the wolf, as the wolf walks away from him, in the background, the name "Pacific Twine and Net" appears on a window pane. (00:15:40)
Revealing mistake: Ruth has gray, curly hair. When Bo Peep bangs The Shepherd's crook on the ground, sending Ruth and David flying to the ground, in the shot from behind them, Ruth's hair is much smoother; the one falling is obviously Gabrielle Rose's stunt double.
Revealing mistake: As Mr. Gold bangs his head on the table after being spritzed with pepper spray, you can see that one performing this action is Robert Carlyle's stunt double, and that the stuntman merely hits the table with his hands. (00:10:20)
Answer: He knows who she is because he knows that the fairy tales are true. He was also shown to sneak out to go see Emma or someone else. Sometimes he succeeded, sometimes he didn't. This is how he knew to look for her despite his strict mother.