When Sam replays Kevin messages on the laptop as Dean is on the couch, after filtering the last message, we hear the last bus stop name. We see Dean getting up, leaning on his left arm. He has a bandage on that arm, from previous cut just below the elbow, that during this "getting up" scene, loosely falls to the wrist onto his watch. Camera changes as Dean finishes getting up and turns to Sam, bandage is miraculously back tight near the elbow.
After the demon possessing Channing slits the roommate's throat, as she begins the spell to make the call, her lips aren't moving for the first two seconds before the scene cuts to the bowl.
In the scene when Sam and Dean are going to visit Kevin's high school girlfriend (Channing), the first shot is of Bascom Hill at the University of Wisconsin- Madison. The red Wisconsin banners are visible on the left and right sides of the shot. Channing is going to school in Centerville, Michigan.
When Dean is performing the tests with Sam to prove they're themselves, they've added soapy water to the routine to show they aren't a leviathan. However this is entirely unnecessary because they cut themselves with a silver blade to prove they aren't a shapeshifter, and this would also prove they aren't a leviathan since they bleed black goo.