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Remember when like every band in the 90s had ”hidden tracks” by filling the ends of their albums with silence?
(hexbear.net)
It kind of failed if you had a fancy CD player (or CD-ROM drive) that could show the lengths of tracks, if every other song on an album is 3 minutes long, then the last one probably isn't 30 minutes long.
I don't know what the point of this post is, other than that it sucks when a modern release removes the hidden track 
There would sometimes be hidden songs in the pregap between tracks too. That meant you had to hold the rewind button at the beginning of a song to hear it. Sometimes it was hidden behind track 1.
The only CD I remember having that did this was Factory Showroom by They Might Be Giants but apparently it happened a lot.
The most interesting hidden thing I've seen albums do is sometimes have computer programs. The Christian rock band Prodigal put audio on a hidden groove on their album Electric Eye. If you recorded that onto a cassette and loaded it into a commodore 64, it would display quotes from Einstein and Jesus.
The game CD for Castlevania: Symphony of the Night had a hidden track you could only hear by popping it into an audio CD player
There were a bunch of
musicians putting Spectrum games on albums in the 80s. https://www.theguardian.com/games/2021/jul/07/video-games-on-vinyl-flexi-discs-zx-spectrum