John,
It's almost like the light turns off for the one frame in the QT (maybe more in real rendering?... you said "rerender frameS") I don't think it's a "ghost" I think a light turns off... Now, keep in mind I don't have a current version to play with and try to understand, is it possible a slider for a light inadvertently slides down in one frame... or that you started keying with that one light down and that frame got stuck in the middle? Can you check light sliders for each frame? (Or tell the lighting guy to stay out of the way?)
I agree with VZ on this...
I was looking at the ani and closed its window and was booted out of the forum cause I didn't use my back key... Minor frustration... but
Respectfully,
Charlie