They're not well known cases but my personal cases that I've literally spent years trying to find again but can't.
In elementary school, around 1995, our principal had an Australian (?) or NZ (maybe?) folk singer visit and perform for us. She had two songs my friends and I remember and still sing/quote as inside jokes. One was like, "we are the denizens of the world - that's what we are," and another that said, "You know it's true blue. You know it's true blue. Aussie export [?]" Since it was 1995 and the internet was like brand new - I doubt it's documented online. I've tried searching soo many things and can't find anything.
There was this thread on B9 about bad music like well over a decade ago, and this band was posted called Church of Hate that was kind of like a Cradle of Filth-ish band. There was a Youtube video of them performing at a venue called Hell's Kitchen (if you look them up - the majority of their videos are at this venue) and they played this hilarious song while mall-goth women spread their asses and pressed their boobs together on stage. The song had these hilarious lyrics like, "fuck me with the fist you fuck yourself with. Dirty little bitch, I know you want it," and "spread your fucking asshole - show it to me!" Since then I've tried to find this video again and can't. I can't even find the song by searching the lyrics verbatim. I even listened to every live video of them I could find looking for the lyrics -- and even listened to tons of their recorded material to try and find the same song, but at no avail. I don't know if it's an obscure cover or what.
Back in the Myspace days this Juggalo rapper from the Akron area added my best-friend. His name was DTK (Durrick [sp?] tha Killa). He had these hilarious songs like, "kill yourself you fucking slut. No one gives a fucking fuck. End it all and split your wrists. No one gives a fucking shit." It was all super low production quality. He ended up having a kid who he named Krados (yes, he spelled it that way). My best friend asked him if he still had demo cds for sale, and I think he knew my friend was making fun of him - so he just ignored him. Fast forward - a lot of his newer material is featuring other bad Northeast Ohio rappers and has a weird auto-tuney feel to it, and he wiped a lot of the older material from online. You can still find profiles for him searching him on Google - but none of them have the great oldies like "Valentine's Day," and "Kill yourself."