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)
Revealing mistake: When Hook tries to rip Zelena's heart out, as a blast of magic hurls him into the wall, you can see that the one who actually hits the wall is Colin O'Donughue's stunt double.
Factual error: The episode opens with Emma going to the State Theater in Mpls. in 1989. The theater was closed for renovation at that time, and hadn't shown a movie since 1975. It was the Jesus People church all through the 80's.
Plot hole: To wake Mr. Gold/Rumplestiltskin from his coma, Emma needs an object that touched him before he became the Dark One. Hook shows her a sword, telling her how he held it to Rumplestiltskin's head and taunted him. However, the sword that Hook shows to Emma, is not the sword that he threatened Rumplestiltskin with in "The Crocodile." The sword that he used to threaten him had a unique, golden handle. When Hook and Rumplestiltskin met again a few years later, Rumplestiltskin took the same sword from Hook and dueled him with it. When Rumplestiltskin vanished into thin air after cutting off Hook's hand, he took the sword with him. The sword that Hook shows to Emma, which Emma uses to wake Mr. Gold, has a black handle with a completely different design, and is the sword Hook carried after the other one was taken from him; he can be seen using it in episodes like "Queen of Hearts", "Lost Girl" and "The Jolly Roger." This means that the sword Emma uses to wake Mr. Gold from his coma never actually touched him at all; however, the spell still works.
Other mistake: When Belle and Gold run from Merida in Gold's shop, she shoots an arrow through the door. We see the end of the arrow (with the white feathers) sticking out of the other side of the door. When when Merida pushes the doors open, the end of the arrow is missing, even though the arrowhead end is still sticking out.
Continuity mistake: When Killian Jones, a.k.a. Captain Hook, is talking to Emma on the street, he points to one of his rings and reveals that it belonged to a man who insulted him by calling him "One Hand Jones." According to Hook, he killed the man in front of his wife, and took his ring as a trophy. However, as seen in 2-04 "The Crocodile", Hook was already wearing this ring before he lost his hand to Rumplestiltskin. This is best seen while he and Milah are drinking and gambling at the tavern: In the shot of the dice landing on the table, you can see Hook's hand resting on the table, wearing the same ring.
Revealing mistake: As Hook is chasing King Arthur through the woods, there is a shot of Arthur running, where you can see that the one running is not Liam Garrigan, but his stunt double. In the next shot, Hook is hot on Arthur's heels and you can see that it is Colin O'Donughue's stunt double who is chasing him.
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)
Revealing mistake: When Hook and Mr. Gold are dueling aboard The Jolly Roger, in several shots, you can see that the ones fighting are not Colin O'Donogue and Robert Carlyle, but their stunt doubles. (00:28:25)
Continuity mistake: When Mary Margaret and Henry are being teleported out of Granny's Diner by Nimue, they aren't wearing any outdoor clothing. However, when they emerge at the pond with the others, Henry is wearing a jacket, and Mary Margaret wears a coat and a scarf.
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.
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.