Decades ago, morality watchdogs feared that Satanic messages were hidden in the music and lyrics of rock records. In fact, they were hidden in the records’ matrices (the portion of the vinyl near the center) where nearly-invisible messages can be printed. The Public Collectors site does a nice overview of the hidden-matrice-message tradition:
Black Flag – “Damaged” (SST)
Side A: COMIN’ AT YA – THE NEOLITHIC GELATINOUS THINK
Side B: SIZZLED THAT NEO-ORTHODOXY RIGHT INTO MY UH, AAAH… WHAT THE HELL!
Led Zeppelin – III (Atlantic)
Side A: blank
Side B: Do what thou wilt