Going Home - S3-E11
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. (00:17:00)
Going Home - S3-E11
Revealing mistake: In the convent, when the Shadow knocks Hook off his feet, sending him sprawling onto the ground, the one who actually falls down is Colin O'Donoghue's stunt double. (00:11:10)
Revealing mistake: After Ariel stabs The Evil Queen in the neck with a fork, Ariel and Snow White jump into the ocean to swim away. In the underwater shot, you can see that the ones swimming through the water are not Ginnifer Goodwin and JoAnna Garcia Swisher, but their stunt doubles. (00:29:20)
Continuity mistake: As Pan (in Henry's body) is first looking through Henry's storybook, after he turns the page, the left page is just text. Cuts to a different angle, and the same page has a big picture on it. (00:13:10)
Snow Drifts - S3-E21
Audio problem: As past Hook carries Emma aboard his ship, he drunkenly and loudly declares, "Behold! The Rolly Joger." Although they are several meters away from the camera, you can see that his lips are barely moving. (00:32:15)
Continuity mistake: After Emma pulls Hook out of the tub of water, the wet hair plastered to his forehead keeps changing from shot to shot. (00:11:30)
The Heart of the Truest Believer - S3-E1
Continuity mistake: After Neal wakes up on the bier, after he says "A .45 caliber arrow", Aurora's dress keeps alternating between having straps and being strapless between shots. (00:13:25)
Other mistake: In "The Heart of the Truest Believer", Rumplestiltskin has his shadow removed from his body, and therefore should not cast a shadow. But in Neverland, his character frequently casts a shadow, especially in scenes with campfires.
Continuity mistake: When Emma and Hook kiss, in the shots from behind Hook, Emma tilts her head to the right. In the shot from behind Emma, she tilts her head to the left. In addition, Emma's left hand jumps from the back of his head to his shoulder between shots. (00:37:10)
Continuity mistake: In Neverland, when Rumplestiltskin's father tells him to think of a food, a slice of cake appears in the palm of the boy's hand. When Rumple walks to place the cake down, suddenly there's a round, white doily under the slice, but when he sets the cake on the wood the doily is gone again. (00:16:50)
The Heart of the Truest Believer - S3-E1
Continuity mistake: When Neal and Mulan are walking to Rumpestiltskin's castle, after Mulan says "Your belief in love wasn't strong enough to overcome your fear of rejection", her hair keeps changing. In close-ups, some of her hair is over her shoulders; in the wideshots, all of her hair is behind her shoulders. (00:23:00)
New York City Serenade - S3-E12
Plot hole: When talking to Hook about the photos, Emma states he could have photoshopped them to make it look like she and Henry were in Storybrooke, however she had taken them directly from Henry's camera and had them developed. So Hook had no way to photoshop the pictures from inside the camera. (00:27:00)
Snow Drifts - S3-E21
Continuity mistake: Aboard The Jolly Roger, when Emma is keeping past Hook occupied while future Hook skulks around the room, in a wide shot, future Hook sneaks past a corner. In the next shot, he begins to sneak past the very same corner again.
Snow Drifts - S3-E21
Continuity mistake: When Emma is making out with past Hook while future Hook skulks around the room attempting to stay hidden, in the second shot, Emma's back is facing the stairs/ladder, her hand gripping Hook's coat. In the next shot, her back is facing the table, and her hand is on Hook's shoulder. (00:33:20)
Snow Drifts - S3-E21
Continuity mistake: While Henry and his family are gathered around the table at Granny's, the amount of juice in the glass sitting on the table keeps rising and falling. (00:06:50)
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)
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)
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)
Revealing mistake: When David is letting Henry drive his truck, there is a close-up of Henry, followed by a wide-shot of the car where the tires are screeching, and you can see that the one who is actually driving is Jared Gilmore's stunt double. (00:30:15)
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.