Almost no one remembers Breakin’ 2. But everyone remembers the title–or, more specifically, the subtitle: Electric Boogaloo. It’s the go-to punchline for ridiculous sequels, as Wikipedia notes (and as you can see on the SDMB if you search for thread titles with “boogaloo” in them).
The 1997 version of Island of Dr. Moreau is basically remembered for being a fiasco. But it also had a wee little sidekick for Marlon Brando, which in turn inspired long-running gags on South Park and the Austin Powers films.
I know there’s more stuff like this out there – little bits from forgotten or seldom-seen movies, TV shows, albums, and other works of pop culture that long outlive their origin. Other examples?