TServo2049 said:
That's not laser rot, it's something on the film. You can tell because it's horizontally stretched due to the anamorphic lens.
Looks like someone accidentally spilled India ink on it. :)
There's a black blob similar to that in ROTJ Gout, at the end of the film as Luke looks down at Leia (lower right part of frame):
http://www.youtube.com/watch?v=3S2auEHR4rg
The frame kind of jerks a bit at that point, as well.