Continuity mistake: When Regina tells Hook about the dark curse, as she says "This new realm", her arm is hanging down by her side. In the next shot, her arm is bent. (00:03:45)
We Are Both - S2-E2
Continuity mistake: When Regina's father, Henry, tells her about Rumplestiltskin, he refers to him as "someone Cora knew before I met her." However, "The Miller's Daughter" establishes that Cora first met Rumplestiltskin while she was locked up in King Xavier's palace, after meeting Prince Henry when she delivered the flour and at the masquerade ball. (00:10:45)
Continuity mistake: When David is having dinner with Regina, as Regina says "How did you like it?" her left hand is lying on the table. In the next shot, her hand is under her chin. (00:20:55)
Continuity mistake: When David is carrying Regina's groceries, there's a plastic bread bag sticking out of one of the grocery bags. The amount of plastic sticking up repeatedly changes between shots. (00:15:45)
Continuity mistake: When Regina is standing by the tree with Daniel, as she says "Have you not seen her magic?" Rocinante, Regina's beloved horse, is standing next to them. In the following wideshot, Rocinante is nowhere to be seen, and there was not enough time to move away from them. (00:08:05)
Continuity mistake: When Regina first kisses Daniel, her head is tilted to the left. In the next shot, her head is tilted to the right. (00:05:05)
Continuity mistake: In the Queen of Heart's maze, when Regina closes the hedge to entrap the pursuers, the spears sticking out from the leaves change position between the wide shot and the close-up. (00:26:10)
Continuity mistake: When the Mad Hatter and The Evil Queen are in the portal of doors, on the left side of the mirror, there's a door with green curtains. The design of the curtains changes between wide shots and the shot where they enter Wonderland. Most notably, in the wide shots, the curtains have brown edges, but when they are standing in front of the mirror, the edges are green. (00:15:55)
Red-Handed - S1-E15
Continuity mistake: In the scene where Ruby reconciles with Granny, in some shots, her necklace is tucked into her clothes. In other shots, it's outside her clothes. (00:36:25)
Continuity mistake: When David is talking to Kathryn in their bedroom, after Kathryn says "It's also been hard on me", the number of buttons that are buttoned on David's shirt keeps changing between shots, which can be seen from the way the opening at the top of his shirt keeps changing in size. (00:34:50)
Continuity mistake: At Granny's, the banknote that Archie puts on the table as he leaves turns around between shots. When Emma asks, "What about that drink?", the money has moved to a different part of the table. (00:28:10)
That Still Small Voice - S1-E5
Continuity mistake: During Henry's first therapy session, as Archie says "So all the crickets in Storybrooke, they were once people too?", Archie's holding a coffee cup in his hand. In the next shot, the cup is sitting on the table. (00:02:45)
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)
Fruit of the Poisonous Tree - S1-E11
Continuity mistake: When Henry is looking for his book at the playground in the beginning of the episode, he drops his bicycle helmet on the ground. In the following shots, the helmet is gone. (00:00:25)
Continuity mistake: When Emma goes to get the fire extinguisher, Regina grabs Emma's arm at the elbow. In the next shot, she's gripping Emma's wrist. In the following shot, she's gripping her elbow again. (00:22:25)
Continuity mistake: In the scene where Morraine is conscripted into the army, as well as the scene where the knights find Rumplestiltskin and his son on the King’s road, Rumplestiltskin's hand repeatedly moves up/down his staff between shots. The same thing happens to Baelfire in the scene where he and his father are preparing to leave for the Duke's palace.
The Thing You Love Most - S1-E2
Continuity mistake: When Emma walks Henry to school, as they exit Granny's, the clock tower building is behind them, and they are walking away from it. But when Henry says "They don't know. It's a haze to them", the clock tower building can be seen on the other side of the street, and they are walking next to it. (00:12:35 - 00:13:10)
Continuity mistake: When Henry and Emma arrive at Storybrooke and Archie Hopper comes over, Archie's scarf alternates between both ends hanging down his shoulders, and one end lying on top of his shoulder/neck. (00:15:55)
Continuity mistake: When Henry is in Emma's apartment, the amount of juice in the bottle repeatedly goes up and down between shots. (00:08:40)
Continuity mistake: When Regina casts aside the guards at Snow White's wedding, one of the guards drops his sword on the floor. In the following overhead shots, the sword has vanished. This is best seen when she turns around after saying "I shall destroy your happiness, even if it's the last thing I do." (00:02:45)
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.