Thanks to Chaotic Chris for figuring out the message, and TDCC1721 for figuring out the letters at the beginning! The message is in the annotations, make sure you have them turned on!
So I don't see anyone else having uploaded this yet, figured I might as well. I don't have a fancy video editor that can reverse video so I did the lazy version and reversed just the audio with audacity and used a picture.
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Do you think this would be considered proof that Gravity Falls will return with a third season...