Continuity mistake: When Henry is sitting at the bar at the end of the episode, reading his storybook, in some shots, there is a picture on one page, which takes up the entire page. In other shots, there is a different picture there, which takes up only half the page and has text next to it. (00:37:25)
Continuity mistake: When Emma and Hook are walking down the street in the beginning of the episode, the time on the Storybrooke clock tower is 8:45 PM. When Mr. Gold is inside the clock tower in the next scene, the time on the clock (seen from inside The Tower) is 8:30 PM. (According to the Blu-ray commentary, the scene with Mr. Gold in the clock tower was originally supposed to take place before the scene with Emma and Hook. This was changed during editing). (00:02:40)
Continuity mistake: When Gideon shows his father the book that Belle gave him, "Her Handsome Hero", in the close-up, the cover of the book is facing upward. In the next shot, where Mr. Gold takes the book, the cover is facing downward.
There's No Place Like Home - S3-E22
Continuity mistake: When Prince Charming shows Snow White the wanted poster, the poster does not have Snow White's name on it. Two shots later, he's still holding it up, but now the poster has her name printed on it. (00:05:30)
Continuity mistake: When King Fergus signs the witch's contract by stamping the hot wax with his ring, the seal is placed right on top of one of the last word of the final paragraph. When the witch shows Merida the contract two years later, the seal has moved several centimeters below the contract. There are also subtle differences in the witch's handwriting. (00:04:45 - 00:06:45)
Continuity mistake: When Zelena is impersonating Mother Superior, she wears a nun outfit with long sleeves. When the real mother Superior enters the room, the latter is wearing an identical outfit, but with the sleeves rolled up. However, in the shot after the real Mother Superior asks "Who are you?" it switches, with Zelena having rolled-up sleeves, and Mother Superior having long sleeves. (00:05:15)
Continuity mistake: When Jefferson hands Emma the scissors, the sharp end is pointing towards Emma. However in the next shot the sharp end is pointing in the opposite direction. (00:24:35)
Continuity mistake: As Dr. Jekyll leads David, Hook, Mary Margaret and Zelena through the marketplace while they try to escape from Mr. Hyde, as Hook says, "Next time, try an aggressive bunny rabbit", he is wearing a ring on his index finger. Two shots later, as they group heads into the alley, the ring is gone (only three seconds have passed, and since Hook only has one hand, he could not have taken it off that quickly). In the next shot, the ring is still missing, and the camera stays on Hook the whole time, but in the next shot, the ring has mysteriously reappeared.
Continuity mistake: When Emma and the others return to Storybrooke, in several shots, like when Regina exits Emma's car, the Storybrooke Cannery appears in the background, painted in white. However, when the magic is released from the Olympian crystal, there is a shot where we see a wave of magic move through Storybrooke Main Street, where the top half of one of the cannery buildings is painted in red.
Continuity mistake: The headstones and statues around Regina's heart vault change dramatically throughout the series. For example, when Emma and Graham find the vault in "The Heart is a Lonely Hunter", there are two headstones on the left hand side of the vault, which are parallel to the side of the vault, and a statue of a woman on the right hand side. When Mary Margaret and David are standing outside the vault in "The Queen Is Dead", there are two completely different headstones on the left hand side, which are perpendicular to the side of the vault. There is also a cross-shaped headstone there, which wasn't there before, and an angel statue, which appeared on the left hand side of the vault when Regina went there in "The Doctor", is gone. When Regina is inside the vault with Mr. Gold in "Welcome to Storybrooke", in the establishing shot, two headstones with a cross engraved on them can be seen near the vault, but these headstones were not there in previous episodes. When Regina goes to the heart vault with a disguised Peter Pan in "The New Neverland", the statues are gone, and there are hardly any headstones around the vault at all. When Emma leaves the vault in "Breaking Glass", the statue on the right reappears, but there are no headstones in front of the vault; however, when Regina is locked inside the vault in "Shattered Sight", there are several headstones in front of it, and the angel statue on the left is back.
Continuity mistake: The location of Regina's heart vault changes throughout the series. For example, when Emma and Graham find the vault in "The Heart of the Lonely Hunter", when Regina and a disguised Peter Pan go to the vault in "The New Neverland", and when Regina hides Zelena's pendant in "Kansas", there is a slope on the left hand side of the building, but when Mary Margaret and David are standing at Johanna's grave in "The Queen Is Dead", when the green smoke goes out of the vault in "Kansas", when Regina is inside the vault with Henry in "The Apprentice", and when Regina is trapped inside the vault in "Shattered Sight", the terrain on the left hand side is flat. The trees around the vault also differ between these scenes.
Going Home - S3-E11
Continuity mistake: When Peter Pan removes the enchanted black leather cuff from his arm, he uses his magic to transfer it to Mr. Gold's wrist. However, as he sends Mr. Gold flying across the room, there is a close-up of Pan's arm, where the cuff is back on his arm, next to the normal brown cuff that he usually wears. (00:21:20)
Continuity mistake: When Will is trying to break into the library, the sign on the door, with the opening hours, is blue. When Will and Robin enter the library, as the door closes, the sign is black. (00:35:15)
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)
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)
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)
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.