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 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)
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: As Emma exits the crypt, in the wide shot, Regina's left hand, holding the flowers for her father's grave, is at her waist. In the next shot, her hand is down by her side, before she brings it up to show the flowers. (00:33:35)
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 with Graham at the Sheriff's office at the end of the episode, her hair keeps changing. For example, in some shots, there is a lock of hair hanging down the left side of her head. In others, it is tucked behind her ear. (00:36:15)
The Heart is a Lonely Hunter - S1-E7
Continuity mistake: When Emma is pressing an ice pack to the wound that Regina gave her, her bracelets repeatedly move up/down her arm between shots. (00:36:25)
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)
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: When Regina is on the stretcher, she has a yellow coat around her shoulders. As she pushes Sidney away, the coat has vanished into thin air. (00:23:50)
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.
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)
True North - S1-E9
Continuity mistake: As Emma returns to the apartment, Mary Margaret is folding clothes on her bed. As she sees Emma, she's holding out her arms, but in the next shot, her arms are down by her side. (00:37:05)
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)
Continuity mistake: The dove that Mary Margaret finds in the woods has white feathers with a reddish plume around its neck. When she releases the bird after the storm has passed, it is not the same dove. The dove that she releases is completely powder white, with a different colored beak. (00:12:20 - 00:29:40)
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)
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 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)
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)
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)
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.