Continuity mistake: When Will is at Marcus' house celebrating Christmas, it's not only the discman box that changes. The box is placed next to the tambourine, but when Fiona stands up to receive Suzie, the tambourine is on the CD player box, and we haven't seen anybody touch it.
Visible crew/equipment: After the math class scene, we see Marcus from behind as he walks toward the bulletin board to sign up for the talent show. In the upper right corner of the frame, there are someone's feet/black shoes and edge of a garment dangling from above. If they don't belong to a crew member, perhaps they belong to Ellie, her part in the scene having been cut out.
Answer: It's when Will is in HMV looking for a CD and it's on the screen in the background.
Hamster ★