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.
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 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 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)
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: 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)
Tougher Than the Rest - S6-E12
Visible crew/equipment: When Sir Henry searches for his mother and The Evil Queen along the water's edge, in the shot where he says, "They crossed the river", as he turns his horse around, a horse wrangler's hand can be seen holding the horse's bridle in the lower half of the screen, guiding the horse to turn around. (00:04:20)
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.
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 Emma lands and Hook land in the apartment after going through the portal, Henry and Regina help them both to their feet. In the first shot, Regina is seen holding onto Hook's arm as he and Emma get up. In the next shot, she is holding onto Emma's arm. (00:34:45)
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)
The Final Battle: Part 1 - S6-E22
Other mistake: While Killian and David are talking at the base of the beanstalk, in the wideshots we can see the protective black boot worn by Colin O'Donoghue, the actor playing Killian, who broke his left foot during filming of the previous musical episode. (00:20:30)
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)
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)
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 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)
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.