OK, this is definitely an obscure one but let it be known that the only time I’ve heard #rcrumb mentioned in a song is this one by ‘70s “Canterbury scene” archetypes Hatfield and the North (“Fitter Stoke Has a Bath”) circa 1975:
“You asked me for a chocolate bar
Then you went and spoiled it all by eating it
What a pain, what a nause!
You can imagine my delight
Was like some R. Crumb magazine come to life
Thank you ladies you had us all
I hope you both enjoyed it”