Nelson's raccoon
The raccoon is the building's animation to show the building is in use --- like the rat on Moe's, the window curtains blowing at the Simpsons house, lawn sprinkler at Flanders, smoke/steam at the powe...
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The animal is gnawing on the protected electrical wires while Nelson plays. The flash you see is the raccoon finally biting through to the wire as Nelson concludes his performance.
