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)
Continuity mistake: The blade on the Dark One's Dagger has one side with the name of the Dark One inscribed, while the other side is decorated with a pattern. Throughout the entire series, the name keeps switching from one side of the blade to the other. In some scenes, the writing is right side up when the dagger is held in the left hand and facing the wielder, with the first letter of the Dark One's name next to the hilt: This happens, for example, when Rumplestiltskin finds the dagger in "Desperate Souls", when Mr. Gold gives the knife to August in "The Return", when Gold summons the wraith in "Broken", when Cora has the dagger in "The Miller's Daughter", when Zelena controls Mr. Gold in "The Tower", when Gold triumphantly shows the dagger to Zelena in "Kansas", when Belle controls Mr. Gold with the dagger in "Heroes and Villains", when Emma summons the first Dark One in "Nimue", when Mr. Gold shows Emma the new version of the dagger in "Swan Song" and when Weaver is holding the dagger at the Hyperion Height police station in "Eloise Gardener." In other scenes, the characters are right side up when the dagger is facing the wielder and held in the right hand, with the first letter near the tip of the blade: For example, when Rumplestiltskin summons Zoso in "Desperate Souls", when Cora steals the knife in "The Queen Is Dead", when Gold uses the dagger in "Going Home", when Regina picks it up in "Kansas", when Gold hides the dagger in "Snow Drifts", when Gold opens the sorcerer's hat box in "A Tale of Two Sisters", when the dagger is lying on the stone in Emma's basement at the end of "Nimue", when the potion is poured onto the blade in "Strange Case" and during the ritual in the cave in "The Guardian."
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: In beginning of the scene where Regina pays a visit to the Sheriff Department, the darts on and around the dartboard change position between the close-up of the dartboard, and the wide shot where Emma bends down to pick up a dart. (00:22:05)
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)
The Shepherd - S1-E6
Continuity mistake: When the twin brother is getting ready to slay the dragon, you can see the scar on his face which hasn't happened yet. In scenes before and after, the scar is not visible. (00:24:00)
Other mistake: At the end of the episode, when Aurora tells Emma and Snow about Henry in her dream, she says that he said his name was Henry, but he never said his name. He just said, "Don't be scared."
Continuity mistake: Leroy is walking by and Astrid she accidentally dumps glitter on his head. She wipes his face off; he has glitter on his moustache. It disappears. The camera pans back to him and there is glitter on his moustache again.
Continuity mistake: When Henry is in New York and trying to conjure magic by the fountain, you can see that the building in the background says Vancouver Art Gallery on the top. (00:23:15 - 00:24:57)
The Garden of Forking Paths - S7-E3
Continuity mistake: In this episode, Henry, Regina and Nook (New Hook), arrive to meet up with Cinderella to join the resistance against Lady Tremaine. In the previous episode, Emma and Hook announced they were expecting a child. No issues there, except that in the last episode of the season Henry and Cinderella have Lucy who is about 10 years old, but Emma and Hook make an appearance with a newborn baby girl. Their daughter should be older than Lucy, since she was pregnant before Henry even married Cinderella. (00:11:13 - 00:38:27)
Revealing mistake: Hook's wire is visible as he climbs in Rapunzel's castle. (00:10:05)
Welcome to Storybrooke - S2-E17
Continuity mistake: When Henry is at the wishing well, intending to blow it up, his backpack keeps moving around throughout the scene. In some shots, it's lying on the ground by the pedestal, in one shot, it's lying on the pedestal, next to the well, and in other shots, it's gone completely. (00:34:20)
Revealing mistake: When Hook is running from the Lost Boys, as he clambers down a hill, you can see the face of Colin O'Donoghue's stunt double. (00:00:50)
Continuity mistake: When Alice and Mr. Gold are performing the magic ritual in the cave, the name on the Dark One's Dagger is misspelled "RumpELstiltskin". In every other episode where the dagger appears, it is spelled "RumpLEstiltskin." (00:30:25)
Revealing mistake: When Detective Rogers is chasing Naveen, in several wide shots, the person running isn't Colin O'Donoghue, but his stunt double. (00:25:50)
The Thing You Love Most - S1-E2
Continuity mistake: When Emma and Henry are walking down the street, the clock face on the clock tower is on the right hand side of the library sign. In the episode's opening scene, and in every other episode, the clock face is directly above the sign. (00:01:15 - 00:13:10)
Revealing mistake: When Emma and Hook arrive at Regina's coronation with their infant daughter, as they enter the room, Hook is holding a doll and not a real baby. (00:38:25)
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)
Continuity mistake: When Mary Margaret drops by Emma's house to discuss wedding plans, in the wide shot where Emma's phone rings, Emma's flower tattoo (which can be seen in several other shots throughout the scene) is missing from her wrist. (00:04:15)
Continuity mistake: When Hook tells Emma in the Sorceror's Mansion about his brother Liam's redemption and eventual moving on from the underworld, his black stud earring isn't on his ear. During that entire scene, the earring disappears and reappears numerous times as the camera cuts between Hook and Emma even though the shot is filmed from the same angle.
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.