When Nathan returned Dogust to his professor you can see a French Fry on his sweater and when he turns to leave it's gone.
Early in the episode, Brooke is wearing boots when she's in the bathroom, but when she runs into the living room in the next scene she is barefoot.
When Nathan looks behind the couch at the dog on the skateboard, he's smiling, but when the camera changes angles his face immediately changes to a serious expression.
While at the tattoo parlor, Alex says that she got her "Believe" tattoo because she survived her suicide attempt in Season 7. However, she's had the tattoo since she first arrived on the show at the beginning of that season. You can see it most noticeably when she's modeling for Clothes Over Bros in her second episode.
Haley rubs the phone number on Brooke's leg with her thumb, but she licks her index finger to determine that the number is written in chocolate.