Here are some suggestions to work with now that the eyebrows has been settled.
Throughout the entire scene when C-3PO and R2 are in front of Jabba’s palace gate, overhead stagelights and the other side of the set are visible in the reflection of C-3PO’s head.
Most particularly in this close up shot where not only the stagelights but also the film crew are clearly reflected from C-3PO’s head
When C-3PO and R2 appear in Jabba’s throne room, C-3PO goes down a flight of stairs with R2 following him but keeps on going. Then when he is approaching Jabba, R2 appears from the lower right corner.
Also, when C-3PO AND R2 are approaching Jabba R2 has his third leg out; then when it cuts to the next shot it’s no longer deployed.
During the Sarlacc battle when R2 bumps into C-3PO you can see several wires actually pulling him as he falls.
The best solution would be to erase it. In this scene when Luke and Leia swing from the sail barge to the skiff, the birthmark is not visible here which shows that this is a stunt double.
When the Emperor arrives in the second DS and is speaking to Vader and says “Everything is preceeding as I have forseen” they are walking past a row of grey-suited officers. Then when it cuts to a wide-angle shot, there’re now in front of rows of stormtroopers.
When Han and Lando are talking to each other in the rebel hanger right before they depart, Lando’s gun belt across his chest changes sides during this scene showing that these shots were flipped.
When Han is running from the shield generator that’s about to explode yelling ‘MOVE!’, his reflection is visible behind him on what appears to be either a plexiglass or camera lens.