Tallahassee - S2-E6
Continuity mistake: Sidney's newspaper article in 1-8 "Desperate Souls" stated that Emma was arrested in Worcester, Massachusetts in 1996. However, this is directly contradicted in this episode, which shows that Emma was actually arrested in Portland, eleven years before the frame story, meaning that the arrest took place in 2001 (since "Selfless, Brave and True" establishes that Emma arrived in Storybrooke in October 2011, and the frame story of "Tallahassee" takes place a few months later, in 2012; the year 2001 is confirmed in 3-22 "There's No Place Like Home"). (00:17:55)
Tallahassee - S2-E6
Continuity mistake: In the giant's treasure room, there are many giant gold coins near the skeleton of Jack the Giant Killer. The amount and position of the coins repeatedly changes. For example, when they find the skeleton, there are several coins on the edge of a small dais, but when Emma saves Hook from the trip wire, they are missing. (00:24:40)
Tallahassee - S2-E6
Continuity mistake: When Emma is arrested, she has a flower tattoo on her left wrist. When she's climbing the beanstalk with Hook, in a few shots, her wrist is bare, and you can see that the tattoo is missing; this happens, for example, when she remarks, "You should be used to people not trusting you." (00:09:10 - 00:29:30)
Continuity mistake: At Lake Nostos, when Emma is pinned down by Hook, the ground beneath her changes between shots, with regards to the markings in the sand (from the fight), as well as the amount of stones, twigs etc. (00:29:00)
Continuity mistake: When Regina is standing in front of Rumpelstiltskin's mirror at the end of the episode, as he asks, "What are you gonna do now, then?", the cloth that was used to cover the mirror is lying on the floor. When he transforms her back to her normal form, it is gone. (00:38:45)
Continuity mistake: When Mr. Gold and Belle are walking to the pawnshop together, their position in relation to the storefronts they walk past keeps changing. Most notably, they walk past a couple of wheelbarrows three times in consecutive shots. (00:13:35)
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 Mary Margaret and Emma attack Cora, she vanishes into thin air. In the next shot, they run toward the portal, and their weapons have mysteriously disappeared. They were there in the previous shot, and there was no time to get rid of them, especially not Mary Margaret's quiver, which was strapped to her body. (00:30:50)
Continuity mistake: During the showdown at Lake Nostos, Mulan's long ponytail alternates between hanging down her back to being in front of her shoulder between shots. Emma's hair also keeps changing position. (00:27:30)
Continuity mistake: When Anton is trapped in the hole, right before Charming exits his car, there is a shot of Anton clinging to the pipe, where you can see the rope they will use to rescue him, dangling next to him; even though the rope hasn't been lowered down yet. (00:20:40)
The Doctor - S2-E5
Continuity mistake: When Hook is tied to the tree, Emma raises her right hand to her mouth to whistle for the ogres. In the next shot, she uses her left hand. (00:26:45)
Welcome to Storybrooke - S2-E17
Continuity mistake: When Henry is at the wishing well, intending to blow it up, his backpack keeps moving around throughout the scene. In some shots, it's lying on the ground by the pedestal, in one shot, it's lying on the pedestal, next to the well, and in other shots, it's gone completely. (00:34:20)
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)
Tallahassee - S2-E6
Continuity mistake: When Emma jumps from the beanstalk and hits the ground, Mary Margaret turns her head to look at her. In the next shot, she turns her head again. (00:38:20)
Continuity mistake: When Belle is watching TV at the Storybrooke hospital, her hair, hanging in front of her shoulders, changes position between the wide shots and the close-ups. (00:10:20)
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)
And Straight on Till Morning - S2-E22
Continuity mistake: When Baelfire attacks Hook, as Mr. Smee is heard saying "captain", Smee runs a few steps, before putting his hands on the railing. In the next shot, he comes running again, and once again puts his hands on the railing. (00:26:25)
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.