Darth Lucas said:

When Luke reaches for the control board "What's that flashing?" and Han slaps him. That should happen numerous times throughout the film.

But "flashing" could be replaced with a different object.

Example: "What's that small moon?"

Another idea, when the officer tries to talk to TK-421, then looks out the window to see Luke (in disguise as a Stormtrooper) he could reply "I can't see a thing in this helmet" as he taps his helmet, or he could say "Huh? Oh the uniform..."