When Rachel tries to persuade Jack to join her team by offering her "cookies" to him, Shawn responds with the familiar phrase: "WHOA!" paying homage to Matthew Lawrence's brother, Joey Lawrence, who played Joey Russo in the TV series, Blossom (1990). "WHOA" was Joey Russo's signature catchphrase in that series. After Shawn says this in the episode, you can see Matthew's sign of approval.
The water stains on Cory's shirt keep changing after Rachel splashes him with water.