Maybe it was a ILM crew member sending a secret message? There is a precedent for it...
Where were you in '77?
Burn Marks - Return of the Jedi Reel 1 - 2 sets:
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Team Negative1
Thank you. Odd that they would manually add a second identifier to the second frame of this change. That second Chewie frames looks almost like it started to burn/blister.
Burn marks from Reel 2, with a diamond over some of them:
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Team Negative1
Here is an image of the blue artifact on Reel 1:
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Team Negative1
Here is an update to the orange marks from Reel 1, and including 1 more possible object:
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Team Negative1
http://i.imgur.com/ffLKtRC.jpg
http://i.imgur.com/UXBSuIE.jpg
http://i.imgur.com/IfTQ5XE.jpg
http://i.imgur.com/whV5ZH9.jpg
1 Orange Marks
2 Blue mark (with Darth Vader)
3 Crack in picture when Luke runs to speeder (Sandcrawler)
4 Stormtrooper with text on his helmet
5 Reel change marks
You might see more, if you search for them.
Team Negative1