Going Home - S3-E11
Other mistake: In the October 2011 flashback, when Mary Margaret gives Henry the storybook for the first time, he opens it to show Pinocchio and Geppetto in an illustration. However, according to 1-20 "The Stranger", Pinocchio's story was not added to the storybook until later after August came to Storybrooke, several months later. This is confirmed in 4-13 "Unforgiven", where Regina and Henry look at the exact same picture, and notice that the paper is different than the rest of the book, because this particular story was added by August. (00:17:00)
The Final Battle: Part 1 - S6-E22
Other mistake: While Killian and David are talking at the base of the beanstalk, in the wideshots we can see the protective black boot worn by Colin O'Donoghue, the actor playing Killian, who broke his left foot during filming of the previous musical episode. (00:20:30)
Other mistake: In "The Heart of the Truest Believer", Rumplestiltskin has his shadow removed from his body, and therefore should not cast a shadow. But in Neverland, his character frequently casts a shadow, especially in scenes with campfires.
Other mistake: At the end of the episode, when Aurora tells Emma and Snow about Henry in her dream, she says that he said his name was Henry, but he never said his name. He just said, "Don't be scared."
Other mistake: When Belle and Gold run from Merida in Gold's shop, she shoots an arrow through the door. We see the end of the arrow (with the white feathers) sticking out of the other side of the door. When when Merida pushes the doors open, the end of the arrow is missing, even though the arrowhead end is still sticking out.
Other mistake: In the end credits, Michael Coleman, who plays Happy, is wrongly billed as "Micahel Coleman." (00:41:00)
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.