The Return - S1-E19
Continuity mistake: When Emma confronts Sidney, the listening device that she places on the table keeps changing position between shots. (00:15:15)
Character mistake: In the newspaper article that Emma reads, Kathryn Nolan's first name is misspelled "Katherine." (00:19:45)
Continuity mistake: When Emma and Jefferson are struggling over the gun, the gun keeps changing position on the floor, without anyone touching it. (00:34:00)
What Happened to Frederick - S1-E13
Continuity mistake: At Lake Nostos, Charming drops his cloak and scarf by the shore. In the following shots, the garments keep changing position on the ground. (00:20:00)
True North - S1-E9
Continuity mistake: When Hansel and Gretel are outside the gingerbread house, Hansel dips his finger in some icing on the wall, which leaves a mark. Two shots later, more of the icing is missing, even though no one touched it. When Gretel climbs onto the window sill, the icing is fully coated. (00:17:50)
Continuity mistake: When Mr. Gold brings the town charter to Mary Margaret's apartment, he puts the binder on the table twice; once before he sits down, and then again after he sits down. (00:19:10)
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: After Emma punches Regina, Regina has a wound by her mouth. For the rest of the episode, the wound keeps changing appearance and position. (00:34:40)
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: When Emma is talking to Mary Margaret in the morning, as Emma says "Tell me, what do I do?", Mary Margaret's left hand jumps from the bowl of sugar to her teacup between shots. (00:09:40)
The Shepherd - S1-E6
Continuity mistake: When Mary Margaret is sitting at the bar with Doctor Whale, the straw and lemon slice in the empty drink in front of her keep changing position between shots. (00:39:40)
That Still Small Voice - S1-E5
Continuity mistake: After Regina tells Emma to clear the crowd away after Henry is rescued from the collapsed mine, Regina's hand goes from being on Henry's face to his side/waist between shots. (00:34:35)
Continuity mistake: When Henry lies on the bed, the position of his legs goes from one leg being crossed over the other, to being parallel to each other, between shots. (00:38:40)
Snow Falls - S1-E3
Continuity mistake: As the trolls search through Prince Charming's belongings, one of them is seen pulling out Snow White's wanted poster from the bag, but in the previous overhead shots, he was already holding it in his hands. (00:27:45)
The Thing You Love Most - S1-E2
Continuity mistake: When Emma is walking Henry to school, there are several instances where they walk past the same building more than once. For instance, as Henry says "Luckily, I have a plan", they pass a shop with a wooden rudder on display in the window. When he says "Don't let her see these pages", they walk past the same shop again. (00:12:40 - 00:14:00)
The Thing You Love Most - S1-E2
Continuity mistake: When Regina is talking to Sidney by the apple tree, as Sidney says "Bounced around all over", Regina is picking an apple. In the next shot, she's already holding it in her hands. (00:11:10)
Continuity mistake: After Charming gives Rumpelstiltskin his cloak, the cloak keeps changing position on Rumpelstiltskin's table. (00:22:05)
Revealing mistake: When Rumplestiltskin is screaming at Snow and Charming about their unborn child's name, he speeds up for a few seconds and looks comically fast forwarded.
Continuity mistake: As Ashley tries to break into Mr. Gold's safe, which is hidden behind a painting, in the first shot, she's moving the painting with her left hand. In the next shot, her right hand is up by the painting, and the lever on the safe has changed position, even though there was no time to turn it. (00:09:55)
Revealing mistake: At the end of the episode, when Emma goes to Gold to ask for his help, we can see he's wearing a wedding ring on his left hand in addition to the usual ring with the light blue stone he wears on the right. That's actually Robert Carlyle's own wedding ring; as he wrote during a twitter Q&A session, he forgot to take it off before they started to film. (00:40:35)
True North - S1-E9
Continuity mistake: When Emma and Mary Margaret are conversing on the bed, Emma's sweater moves up/down her left shoulder several times. For example, as Emma says "Apparently that book you gave him, aren't exactly the stories in the most traditional sense", the sweater is over her shoulder. In the next shot, it's off her shoulder. In the following shot, it's covering her shoulder again. (00:37:45)
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.