Deliberate mistake: Elsa's dress in Season Four is different from the dress she was wearing in the Season Three finale. The new dress is a different shade of blue, and has a completely different design, even though the present day events of Season Four take place mere moments after the end of Season Three.
What Happened to Frederick - S1-E13
Deliberate mistake: When August takes Emma to the Wishing Well, the well is not in the same location as later in the series. When Emma and August are at the well, it is situated next to the road (August takes her there on his motorcycle), with a giant tree in the background. In in 1-22 "A Land Without Magic", and every other episode where the well appears, in is situated in a clearing in the middle of the forest. (00:22:35)
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.