This article is from the The Simpsons FAQ, by Ondre Lombard wakkojr@nospam.wcat.com and Andrew Gill superluser@mail.rit.edu with numerous contributions by others.
Each episode is produced with extra and alternate animation, to allow the
directors the freedom to decide just what will and won't work, up until a
few days before a show goes to air. Fox has access to the footage before
the final cuts are made, and so it's not unusual for extra material to
materialize during the promotions.
Also, the Fox network has taken to mixing clips from previous episodes
with clips from upcoming episodes, supposedly to make episodes more
enticing or exciting. The best example of this is the commercial for
"The Itchy and Scratchy and Poochie Show" in which Homer's character
Poochie is on TV and Lisa says "The network has sunk to a new low,"
the footage coming from 9F22 "Cape Feare."
 
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