What Happened to Frederick - S1-E13
Deliberate mistake: When August takes Emma to the Wishing Well, the well is not in the same location as later in the series. When Emma and August are at the well, it is situated next to the road (August takes her there on his motorcycle), with a giant tree in the background. In in 1-22 "A Land Without Magic", and every other episode where the well appears, in is situated in a clearing in the middle of the forest. (00:22:35)
What Happened to Frederick - S1-E13
Revealing mistake: When Regina finds Henry with Emma at his playground In "Fruit of the Poisonous Tree", her license plate number is 40F5 TH. When the gym teacher finds Kathryn's car in this episode, his car has the same license plate, indicating that it is a reused prop. The same license plate appears on yet another car when August drives off with his motorcycle in "The Return", on Archie Hopper's car when he tries to cross the town line in "We Are Both", and on a parked car when Emma and the others arrive in Storybrooke in "Mother." (00:40:10)
What Happened to Frederick - S1-E13
Revealing mistake: At Granny's, when Emma and August ride off on the motorcycle, you can see that the one driving is actually Eion Bailey's stunt double. (00:21:45)
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)
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.