Plastic Underwear was the Butterfinger commercial that originally aired on December 31, 1999.
Lisa bites into her Butterfinger, awakening a sleeping Grampa. When he attempts to go back to sleep, the biting noise happens again, though this time it's not Lisa but Grampa's plastic underwear. Once Grampa is back sleeping Lisa once again bites her Butterfinger.
(Originally, Grandpa was mentioning that it was his osteoporosis, but he was overdubbed in this version.)