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)
Snow Drifts - S3-E21
Continuity mistake: When Emma changes clothes in the Enchanted Forest, as she puts on gloves, in the wide shot, she begins to put the glove on her left hand. In the following close-up, her left hand is bare, and she's putting the glove on her right hand. (00:18:15)
Continuity mistake: When Zelena loses the Dark One's Dagger, it lands on the ground, the patterned side of the blade facing upwards. When Regina picks up the dagger, the patterned side is facing downwards. (00:28:10 - 00:29:20)
It's Not Easy Being Green - S3-E16
Continuity mistake: When Hook is at the beach with Henry, after he says to him, "Fair enough, mate", the pendant on Hook's necklace goes from resting against his chest, to being partially tucked under his waistcoat. In the close-ups, the pendant is completely covered by the waistcoat. (00:26:50)
Continuity mistake: On the plane, Henry is eating baked goods from Cinnabon's. The size and position of the stains of frosting inside the box changes after he says "The only thing that could make this day any better, is more frosting." (00:40:25)
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)
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: 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)
Continuity mistake: When Belle is looking for Baelfire's shawl and distractedly sets down the gun, the gun keeps changing position on the bench. (00:24:00)
We Are Both - S2-E2
Revealing mistake: When Mary Margaret tries to escape and Mulan uses a sling to bring her down, you can see that the one who falls to the ground is Ginnifer Goodwin's stunt double. (00:39:50)
Continuity mistake: When Killian rallies his crew to join him in taking over the ship and becoming pirates, as he's holding onto the ropes used to control the vessel's sails, the number of ropes he's grasping repeatedly changes between shots; in some shots, it's only two ropes, in other shots, there are three ropes in his hand. (00:38:50)
Going Home - S3-E11
Continuity mistake: In the Neverland flashback, when Hook offers Tinker Bell the rum bottle, the stopper (attached to the bottle) is hanging over the back of his hand. In the next shot, it is not. (00:09:55)
Save Henry - S3-E9
Continuity mistake: When Regina talks to Rumplestiltskin, he is holding onto the bars in his cell. In the beginning of the scene, his left hand keeps clutching different bars between shots. After Regina says "Nothing can stop me now", his hands keep going from holding the bars and not holding them. (00:02:00)
Continuity mistake: In Neverland, the piece of cake that Rumplestiltskin wishes for changes during this scene, with regard to the amount of chocolate, white cream, etc, and also its position on the wood after Rumple puts it down. (00:16:45)
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)
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)
Continuity mistake: In the scene where Rumplestiltskin comes home and discovers that Milah is not there, the wool he's carrying keeps changing position in the basket. (00:02:10)
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.