A Very Special Family Guy Freakin' Christmas - S3-E16
Continuity mistake: When Lois reminds Peter to go get the tree, there's a wreath with a bow hanging on the wall between the two pictures behind the couch, but when Lois convinces Peter with the promise of a new VCR that wreath has vanished, then it reappears and disappears again when Peter returns with the tree. Also, the plate of cookies that are on the arm of the couch seem to vanish as well.