Revealing mistake: When Killian Jones is dueling Rumplestiltskin and is pushed into some barrels, you can see that the one who actually falls down is not Colin O'Donoghue, but his stunt double. (00:18:20)
Continuity mistake: When Rumplestiltskin rips out Milah's heart, Killian catches her as she falls down. In the first shot, the back of her head is resting on his shoulder. In the next shot, her position has changed, and her head is resting on his arm. (00:28:10)
Continuity mistake: Before the duel, as Rumplestiltskin says, "Sorry, but killing a man with his own sword was just too delicious to pass up", Killian's right arm is bent. In the next shot, his arm is down by his side. (00:17:25)
Revealing mistake: As Killian Jones and Rumplestiltskin duel, there is an overhead shot of them fighting, and in the next shot, you can see that Robert Carlyle and Colin O'Donoghue have been replaced by their stunt doubles. (00:17:35)
Continuity mistake: Aboard The Jolly Roger, Rumplestiltskin says "Do I forgive you? Can I move on?", while walking past Milah. In the first shot, he is right behind her, but in the following wide-shot, he has suddenly moved to a position several meters away from her. (00:26:40)
Continuity mistake: When Milah and Killian board The Jolly Roger, as Milah says "Fetch some water", a pirate with a coil of rope around his arm has just ascended the deck and is seen near the steps. In the next shot, he is nowhere near that area (he can briefly be glimpsed as he disappears behind a mast in the lower left hand corner). (00:25:45)
Continuity mistake: When David and Rumplestiltskin walk down the streets of Storybrooke while searching for Belle, as David says "There is a difference between literal truth and honesty of the heart", a car is seen driving through the street. In the next shot, it's gone. (00:21:20)
Continuity mistake: When David and Rumplestiltskin are looking for Belle, when Rumple says "Look, can I ask you a question?", in the background, a man and a woman are crossing the street. In the next shot, they cross the street again. (00:20:40)
Continuity mistake: When Rumpelstiltskin and David are out looking for Belle, they cross the street, and for the rest of the scene, the pedestrians change from shot to shot. (00:20:55)
Continuity mistake: At Granny's, when Belle and Ruby chat, the slice of lemon and the straw in Belle's iced tea move around between different camera angles. The same thing happens to the straw in the half empty glass in front of her. (00:12:15)
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.