Incredible! Songs in the Key of X was the only album I ever knew to do this, and it wasn't even the first. I had no idea so many others did the same thing.
Edit: Son of a *, I've had a copy of Sister Machine Gun's Burn for almost 30 years and never knew there was a hidden track!
Classic X-Files album is the one I think of too. And how they hinted to everyone that there even was a hidden track on the sleeve: "'0' is also a number". (and the technical fineprint about the disc possibly not being Redbook compliant)
