Continuity mistake: When Bossy is cutting Dreamy's beard, Dreamy asks "Who was the woman I saw?" and Bossy stops cutting, but a couple of shots later, Dreamy's beard is suddenly much shorter. (00:10:00)
Character mistake: In the evening, Emma states that Kathryn Nolan has been missing for 24 hours, meaning that this episode takes place the day after Kathryn disappeared. When Emma investigates Kathryn's car, David tells Emma that he last spoke to Kathryn "yesterday afternoon" (and stresses that he is telling the truth), and that he hasn't spoken to her since he ended their relationship. However, "What Happened to Frederick" establishes that David broke up with Kathryn the day before she tried to leave town and disappeared, because when Kathryn apologized to Regina, Regina referred to the events surrounding the break-up as "what happened yesterday." Emma checks Kathryn's phone records and finds out that Kathryn talked to David on the phone the day she disappeared, but David could not have referred to that conversation, since "Red-Handed" and "Heart of Darkness" both establish that he doesn't remember the phone call because of a memory blackout. Making matters even more complicated is that when Sidney shows up with the phone records, Emma specifically says to him that David told her that he DIDN'T speak to Kathryn that day, which, in fact, he said he did. (00:12:45 - 00:26:45)
Continuity mistake: Leroy is walking by and Astrid she accidentally dumps glitter on his head. She wipes his face off; he has glitter on his moustache. It disappears. The camera pans back to him and there is glitter on his moustache again.
Revealing mistake: When the newly hatched dwarves are being cleaned, there is a shot where Sleepy is seen from behind as he's drying himself with a towel after getting out of the bath tub, and you can see that the actor is wearing black shorts underneath. (00:09:45)
Factual error: In "What Happened to Frederick", Kathryn learns about David's affair and confronts Mary Margaret at school (after taking her anger out on Regina), where students and teachers are milling about. Later, she apologizes to Regina for "what happened yesterday", whereas Emma, earlier that day, seeks out Henry at school. Kathryn leaves Storybrooke the same day, and mysteriously disappears. According to Kathryn's phone records in this episode, she disappeared on a Sunday. However, the day she left, and the day before, was a school day. In the U.S., school classes are not in session during weekends. (00:38:25)
Continuity mistake: When Mary Margaret and Emma are walking down the street, as Mary Margaret says, "This is the best I can do", she begins to walk past the corner of a building. In the next shot, they begin to walk past it again. (00:05:15)
Continuity mistake: In "The Shepherd", Kathryn is showing David their house, and you can see that the street number on the Nolans' house is 244. In this episode, when Emma is looking at Kathryn's phone records in the evening, the phone records list her address as number 4. (00:38:25)
Continuity mistake: After the door to door candle sales fail, when Leroy is about to deliver the bad news to Astrid, the position of Mary Margaret's coat, which she's holding in her arms, changes between shots. (00:21:00)
Continuity mistake: When Astrid accidentally dumps glitter on Leroy, in the first shot, there's hardly any glitter on him. Cuts to the next shot, and he is covered in glitter. The amount of glitter on his clothes repeatedly changes for the rest of the scene. (00:05:20)
Continuity mistake: In the scene where Emma is taking pictures of Kathryn's car and Sidney shows up, they open the trunk lid. None of them close it, and the camera is on them the whole time, but when David shows up, the lid is closed. (00:08:00)
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.