Revealing mistake: The phone message that Ryan leaves on Luke's voicemail is different to what he originally said, which is "I want to make sure you took care of it. Take care of it," but the message recorded is, "I'm just calling to make sure you took care of it. Take care of it." Also the background sounds from the club - the music, people talking, etc, are missing.
Other mistake: In this episode when Marissa and Summer meet Cody Briches on the beach, Marissa says she's never watched "The Valley," which is not true, as in the Valentines Day episode, she's lying on the couch watching T.V and you can hear "Next Time On The Valley."
Plot hole: Ryan says that he has left his watch at the motel after sleeping with Theresa, and goes back to get it. However, at the end of The Goodbye Girl (the episode before) he is seen wearing it as he gets out of the car to go and talk to Theresa, only to find she's gone back to Chino. The whole story of Ryan and Seth going back to get his watch from the motel is just an excuse for them to discover the whole Luke/Julie affair.
Answer: She is referring the episode entitled: The Best Chrismukkah Ever, episode 13, season 1. She goes out christmas shopping with Ryan and gets stopped by a security when she is out by Ryan's car. He asks to look inside her bag and finds a stolen watch and some other stolen things.