From SmashWiki, the Super Smash Bros. wiki
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The SleepSliding is a specific glitch for Pikachu and Jigglypuff in Super Smash Bros.. While on a flat platform, if Pikachu does a horizontal Quick Attack through a singing Jigglypuff, once a hit has registered as Pikachu connects with Jigglypuff, Pikachu will slide along the entire platform until it comes in contact with a ledge.