Continuity mistake: When David is talking to Kathryn in their bedroom, after Kathryn says "It's also been hard on me", the number of buttons that are buttoned on David's shirt keeps changing between shots, which can be seen from the way the opening at the top of his shirt keeps changing in size. (00:34:50)
Continuity mistake: At Granny's, the banknote that Archie puts on the table as he leaves turns around between shots. When Emma asks, "What about that drink?", the money has moved to a different part of the table. (00:28:10)
That Still Small Voice - S1-E5
Continuity mistake: During Henry's first therapy session, as Archie says "So all the crickets in Storybrooke, they were once people too?", Archie's holding a coffee cup in his hand. In the next shot, the cup is sitting on the table. (00:02:45)
Continuity mistake: In the scene where Emma is taking pictures of Kathryn's car and Sidney shows up, they open the trunk lid. None of them close it, and the camera is on them the whole time, but when David shows up, the lid is closed. (00:08:00)
Fruit of the Poisonous Tree - S1-E11
Continuity mistake: When Henry is looking for his book at the playground in the beginning of the episode, he drops his bicycle helmet on the ground. In the following shots, the helmet is gone. (00:00:25)
Continuity mistake: When Emma goes to get the fire extinguisher, Regina grabs Emma's arm at the elbow. In the next shot, she's gripping Emma's wrist. In the following shot, she's gripping her elbow again. (00:22:25)
Continuity mistake: In the scene where Morraine is conscripted into the army, as well as the scene where the knights find Rumplestiltskin and his son on the King’s road, Rumplestiltskin's hand repeatedly moves up/down his staff between shots. The same thing happens to Baelfire in the scene where he and his father are preparing to leave for the Duke's palace.
The Thing You Love Most - S1-E2
Continuity mistake: When Emma walks Henry to school, as they exit Granny's, the clock tower building is behind them, and they are walking away from it. But when Henry says "They don't know. It's a haze to them", the clock tower building can be seen on the other side of the street, and they are walking next to it. (00:12:35 - 00:13:10)
Continuity mistake: When Henry and Emma arrive at Storybrooke and Archie Hopper comes over, Archie's scarf alternates between both ends hanging down his shoulders, and one end lying on top of his shoulder/neck. (00:15:55)
Continuity mistake: When Henry is in Emma's apartment, the amount of juice in the bottle repeatedly goes up and down between shots. (00:08:40)
Continuity mistake: When Regina casts aside the guards at Snow White's wedding, one of the guards drops his sword on the floor. In the following overhead shots, the sword has vanished. This is best seen when she turns around after saying "I shall destroy your happiness, even if it's the last thing I do." (00:02:45)
Continuity mistake: When The Evil Queen crashes Snow White's wedding, the guests alternate between looking at the queen and averting their gaze between shots. (00:02:30)
Continuity mistake: When Sean is visiting Ashley at the hospital, as he says "Yeah, I'm back", baby Alexandra is waving her tiny hands in the air. In the previous and following shots, her hands are down. (00:39:10)
Continuity mistake: After Henry runs up the stairs, he puts his backpack on the bed, with the backside facing upwards. When he takes off his shoe and throws it on the floor, the top of the backpack is facing upwards. When Regina enters his bedroom, the backside is facing up again. (00:38:35)
Continuity mistake: When Ashley is holding baby Alexandra while Emma comes in, her left hand repeatedly changes position between shots; for example, when Emma says "It's pretty", her hand is on one side of the baby's head; in the next shot, it is on the other side. (00:36:25)
The Thing You Love Most - S1-E2
Continuity mistake: Throughout the scene where Emma walks Henry to school, the surroundings change between shots. For example, when Emma asks, "Because I'm the daughter of Snow White and Prince Charming?", they are in front of a clock store. In the next shot of Emma, there's a completely different shop behind her, even though neither of them move. (00:12:35)
Fruit of the Poisonous Tree - S1-E11
Continuity mistake: When Emma takes out Sidney's card at Henry's playground, in the first shot, her hand is closed around the bottom of the card. In the next shot, she's holding it between her thumb and index finger, with the thumb on top of the card. (00:07:10)
Fruit of the Poisonous Tree - S1-E11
Continuity mistake: When Sidney hands Emma a card with his number, the way he's holding it in his hand keeps changing between shots. (00:02:40)
Continuity mistake: After Charming and Snow embrace in King George's castle, Snow's hand is on Charming's chest. In the next shot, it is resting on his shoulder. (00:32:45)
Continuity mistake: When Mary Margaret is talking to Regina in the school yard, her fringe keeps changing between shots. (00:08:00)
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.