Continuity mistake: While Emma is talking to Mary Margaret on the phone, an extra (lady with a purple shopping bag) comes from behind Emma. The scene then switches to Mary Margaret, then back to Emma, and we see the exact same extra walking behind her again. (00:14:40)
Continuity mistake: When David is making breakfast, he turns around, with a frying pan full of pancakes in his hands. In the next shot, he begins to lift up a pancake with a spatula, but when it cuts, he's already holding it out. (00:03:15)
Continuity mistake: After Mary Margaret opens the birthday present from Johanna, the wrapping, lying around the present, changes position a couple of times. (00:03:55)
Welcome to Storybrooke - S2-E17
Continuity mistake: When Emma is having breakfast, the amount of cereal in the bowl in front of her keeps changing. (00:09:55)
Welcome to Storybrooke - S2-E17
Continuity mistake: During breakfast in Mary Margaret's apartment, there are two plates of toast on the bench. The distance between the plates changes between different camera angles. In some shots, they are right next to each other; in others, there is a gap of approximately 30 centimeters between them. (00:09:55)
Welcome to Storybrooke - S2-E17
Continuity mistake: In Mary Margaret's apartment, when Emma is having breakfast with her family, after she says "He deserves to know", her right arm is resting on the counter, next to the bowl of cereal. In the next shot, both of her arms are resting in her lap. A spoon in the bowl also changes position. (00:10:25)
Welcome to Storybrooke - S2-E17
Continuity mistake: When Kurt and Owen are in the diner, in the first shot, Kurt is holding his tankard of beer by gripping the top. In the next shot, he's holding it by the handle, and the vegetables on his plate have changed position. (00:11:55)
Welcome to Storybrooke - S2-E17
Continuity mistake: Baelfire/Neal is in the diner and wraps his scarf around his neck while Emma walks over to him. Then the camera is on Emma and you can see Baelfire/Neal moving as he puts the scarf on. Seconds later the camera switches back to Baelfire/Neal and he wraps the scarf around his neck a second time. (00:25:05)
Welcome to Storybrooke - S2-E17
Continuity mistake: When Henry is in the forest while it's raining, his hair keeps changing, especially his fringe. (00:28:20)
Welcome to Storybrooke - S2-E17
Continuity mistake: When Greg Mendell bumps into Henry in the woods, Henry drops his backpack on the ground, the backside facing upwards. When Greg picks it up, the front is facing upwards. (00:28:40)
Welcome to Storybrooke - S2-E17
Continuity mistake: When Henry is at the wishing well, intending to blow it up, his backpack keeps moving around throughout the scene. In some shots, it's lying on the ground by the pedestal, in one shot, it's lying on the pedestal, next to the well, and in other shots, it's gone completely. (00:34:20)
Continuity mistake: When Mr. Gold and Lacey are on a date at Granny's Diner, when Granny comes with their drinks, she moves her hand towards Gold's glass of ice tea, sitting on the tray. In the next shot, we hear the clinking of ice as Granny picks up the glass, as well as the sound of the glass being put on the table, off-screen, on Gold's left. In the next shot, the glass has mysteriously moved to Gold's right, even though the camera was on him the whole time, and he never moved it. (00:25:00)
Continuity mistake: In the scene where the Sheriff of Nottingham (his Storybrooke counterpart) tries to apologize to Mr. Gold, they're at the corner of the Rabbit Hole, and in some shots, the clock tower building is seen behind the Sheriff. In other shots, there's a completely different building behind him. In the same scene, Mr. Gold alternates between standing right by the corner, and standing a couple of meters away. (00:35:40)
Continuity mistake: When Regina is standing in front of Rumpelstiltskin's mirror at the end of the episode, as he asks, "What are you gonna do now, then?", the cloth that was used to cover the mirror is lying on the floor. When he transforms her back to her normal form, it is gone. (00:38:45)
Second Star to the Right - S2-E21
Continuity mistake: When Baelfire eats the bread, what remains of the loaf keeps changing shape and position on the tray between shots, and the two other loaves also move around. When Wendy picks up the tray, all three loaves are untouched. (00:02:20)
And Straight on Till Morning - S2-E22
Continuity mistake: At the beginning of the scene where Baelfire confronts Hook about Milah, when Hook is talking to Smee, the hair on his forehead keeps changing between consecutive shots. (00:25:55)
And Straight on Till Morning - S2-E22
Continuity mistake: In the scene where Mr. Smee asks Hook why Baelfire is still aboard The Jolly Roger, when Hook says "Anyone who disobeys can walk the plank and pray the mermaids take pity on his soul", as he begins to walk down the steps, Smee is several meters away from that area, with his back turned (look behind the mast). In the next shot, he is much closer to the steps, and is facing the viewer. (00:26:20)
And Straight on Till Morning - S2-E22
Continuity mistake: When Baelfire attacks Hook, as Mr. Smee is heard saying "captain", Smee runs a few steps, before putting his hands on the railing. In the next shot, he comes running again, and once again puts his hands on the railing. (00:26:25)
And Straight on Till Morning - S2-E22
Continuity mistake: When Baelfire is getting ready to leave The Jolly Roger, his right hand, which is holding onto the sack he carries behind his back, keeps moving up/down between shots. (00:29:15)
The Heart of the Truest Believer - S3-E1
Continuity mistake: After Neal wakes up on the bier, after he says "A .45 caliber arrow", Aurora's dress keeps alternating between having straps and being strapless between shots. (00:13:25)
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.