Star Trek II: The Wrath of Khan - The Mysterious Moving Bloody Handprint
When Scotty's nephew (Ike Eisenmann) is in sick bay, and thereafter, the bloody handprint that he leaves on Kirk's jacket lapel (the white part) moves, changes size, changes shape, etc. It never seems to be the same print twice in a row.
This one is fairly obvious, so I'm sure you've all seen it.
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|This a picture of the stain in the first scene it is in.
||This a picture of the stain in the second scene it is in.