Sam says that changelings will feed off the mother's synovial fluid, and after feeding on them for a few weeks the mother dies. However synovial fluid is only found in joints, it helps lubricate and cushion the joint. Draining a person of all their synovial fluid would cause their joints to be extremely swollen and painful, but it would not kill them.
Indiana doesn't issue front license plates.
When Katies dad was in his garage he was working on a rocking horse. When he left the horse only had three of its legs. After he got killed the rocking horse is shown standing up rocking and it has all four legs.
At the start of the episode, when Dean leaves Sam to first visit Lisa, there is a flyover shot of the neighborhood. Clearly visible are Canada Post mailboxes, which wouldn't be present in Indiana.