We're guessing the reason for Chewbacca's mistake in the original movie is that droid heads are not only so modular that they can be replaced onto any droid body, but were also designed by the same people who designed USB plugs, so that you never quite know if you're inserting them the right way around or not until you try it.
C-3PO: Oh, it's good to have my head on properly again.
R2-D2: No problem.
[SFX]: < boodoot >
C-3PO: You might not realise, but when it was on backwards I kept saying the opposite of what I meant!
Leia: What, really? I had noooo idea.
Chewbacca: Yes, my apologies, 3PO. At the time it was necessary to conceal Han's and my treachery from Princess. To no avail, it appears.
Leia: Ha ha, you guys! You total traitor pants!
Lando: You're letting him get away with that?
Leia: Noooo... We need to decide between the space lions or the laser-rack. Or the plasteel maiden.
Lando: After a fair trial, of course.
Leia: But who's gonna volunteer to try those?