According to Once Upon A Galaxy, it's the little scene where Luke is suiting up in the Medical Center and chatting with 2-1B before the big battle on Hoth. Kershner asked Lucas to help out after catching him peering through a camera viewfinder. Lucas allegedly did many takes.
Once Upon A Galaxy is such a great snapshot of what it was really like on the Empire set. I'm really hoping they follow up that recent making of Star Wars book, and reprint it. My copy is falling apart!