Continuity mistake: At Lake Nostos, in the first shot of Emma, Snow and Mulan, there is a giant rock on their left. None of them move, but two shots later, the rock is on their right.
Continuity mistake: At Lake Nostos, when Emma says "You're not going anywhere. This portal's taking us home", in the wideshot, Snow's quiver strap is over her pink cardigan. In the close-up, it is under the cardigan. In addition, Mulan's and Snow's weapons are held at different angles between shots.
Tallahassee - S2-E6
Continuity mistake: When Emma cuts the trip wire in Anton's castle, the surroundings change completely between shots; for example, as she cuts the wire, there is an open chest near her. In the overhead shots, the chest is gone, and Jack's skeleton is missing. (00:33:50)
Continuity mistake: When Snow is dumped by the road by King George's men, the amount of dirt on her clothes keeps changing between shots. (00:14:15)
Tallahassee - S2-E6
Continuity mistake: As Snow and Mulan fight, Snow's pink cardigan gets covered in straws. The amount and position of the straws keep changing during the fight, and afterwards. Also, when Snow tackles Mulan, in the first close-up of them fighting, you can see straws from previous takes on Snow's cardigan, even though she has not come into contact with the ground yet, and the cardigan was perfectly clean in the previous shots. (00:37:40)
Tallahassee - S2-E6
Continuity mistake: When Anton throws the compass to Emma, the compass changes position on the floor between shots, seen from the lines/cracks in the floor. (00:33:45)
The Doctor - S2-E5
Continuity mistake: When Emma is questioning Hook while the group is gathered around the bench, the folds in the cloth/bundle lying on the bench keep changing between shots. (00:14:30)
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: 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)
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.