Continuity mistake: When Red meets Quinn, the way her hood is draped over his arm changes throughout the scene. The same thing happens when Red is holding it afterward. After Quinn drops the hood, the way the garment is crumpled up on the ground changes between shots. (00:12:05)
Continuity mistake: When David is confronted by Albert Spencer at the diner, as David gets up, his glass of beer is almost empty. When he picks it up, the glass is half full. (00:05:15)
Continuity mistake: At Granny's, when Ruby is clearing tables and Billy startles her, the amount and position of the food on a plate changes between shots. The cup and the napkins also change position on the table. (00:02:35)
Continuity mistake: When Anita lies dying, Red Riding Hood holds her hand over her mother's chest, in front of the arrow, and Anita grabs her hand. In the next shot, their hands are down at Anita's side, behind the arrow. (00:31:10)
Continuity mistake: In the scene where Albert Spencer burns Jefferson's hat, Ruby's hair alternates between hanging down the side of her face and being tucked behind her ear. (00:35:25)
Continuity mistake: When Albert Spencer confronts David at Granny's, Henry's coffee cup moves to a different part of the table, and back again, between shots. (00:04:40)
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.