Revealing mistake: As Killian Jones and Rumplestiltskin duel, there is an overhead shot of them fighting, and in the next shot, you can see that Robert Carlyle and Colin O'Donoghue have been replaced by their stunt doubles. (00:17:35)
Revealing mistake: When Killian Jones is dueling Rumplestiltskin and is pushed into some barrels, you can see that the one who actually falls down is not Colin O'Donoghue, but his stunt double. (00:18:20)
Tallahassee - S2-E6
Revealing mistake: When Hook and Emma are climbing up the beanstalk, as Hook remarks, "First beanstalk?", you can see that Colin O'Donoghue is actually gripping the vine with his left hand, which is covered up by his sleeve; the hand that was cut off. (00:08:50)
Tallahassee - S2-E6
Revealing mistake: When Neal is being chased by August, he runs on top of a car before attempting to jump the fence. Just before he jumps on top of the car, you can see that the one who actually performs this action is Michael Raymond James' stunt double. (00:26:40)
Revealing mistake: When Belle hits Hook with a rowing paddle, as he falls into the hatch, you can see the face of Colin O'Donoghue's stunt double. (00:28:10)
Revealing mistake: When Hook attacks Mr. Gold, if you watch as he pushes Emma out of the way, you can easily see that one who actually pushes Jennifer Morrison is Colin O'Donoghue's stunt double. When Hook pushes Mr. Gold against the security gate, it is the stunt double who pushes Robert Carlyle. (00:18:25)
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)
Selfless, Brave and True - S2-E18
Revealing mistake: As Tamara opens the envelope of cash in the bar in Hong Kong, the words "motion picture use only" are printed on the dollar bills. (00:18:30)
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)
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)
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: 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)
Snow Drifts - S3-E21
Revealing mistake: When future Hook is trying to stay hidden while Emma is making out with past Hook, there is a wide shot where future Hook sneaks past a corner, and you can see the face of Colin O'Donoghue's body double. (00:33:20)
There's No Place Like Home - S3-E22
Revealing mistake: After Snow White hugs Red Riding Hood goodbye, there is a wide shot where Snow walks toward Charming. If you look at her face, it it not Ginnifer Goodwin, but a body double. The body double's face can also be seen in the wide shot where Snow walks away. (00:19:20)
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: When Mary Margaret and Regina engage in a sword fight in the sheriff's office, in several wide shots, the faces of Ginnifer Goodwin and Lana Parrilla's stunt doubles are visible. For example, in the wide shot where Regina is bending Mary Margaret across the desk and David says, "Careful", Lana Parrilla's stunt double is seen. (00:30:20)
Revealing mistake: When Emma opens the door of Any Given Sundae in the 2011 flashback, you can see the backdrop curtain used to insert the Storybrooke main street, thanks to its shadow in the middle of the road and a few folds reflecting the light.
Unforgiven - S4-E13
Revealing mistake: When Snow White and Prince Charming arrive home at the Royal Castle on horseback, you can easily see that the ones riding are not Ginnifer Goodwin and Josh Dallas, but their stunt doubles. (00:02:55)
Revealing mistake: When Regina is waiting outside the Storybrooke Library, there is a wide shot of Cruella's car making its way down the road. On the building on the right side, you can see a large vertical sign on the side of the building, which lists several local businesses. If you read the top of the sign, you can see that it says "Steveston Hotel." Steveston is the Canadian town which doubles as the town of Storybrooke on Once Upon a Time. (00:20:45)
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.