Vinyl pressing mistakes


Ever seen an LP with the the wrong label? I have an old LP that has a different artist/album side pressed on the b side. I listened to the a side of 'Blue Man' by Steve Khan.... flipped it over and Tim Wiesebeg/Dan Fogalberg (sp?)'Power of Gold' or some such nonsence starts playing... surprise! How common are these mistakes? Just curoius.
blkadr
I just picked up a two-box UK pressing of Revolver that is stereo on side 1 and mono on side 2. The jacket and labels say stereo but the matrix numbers are YEX for the first side and XEX for the second. I had to check after wondering why there wasn't any weird panning on the first few songs. The stereo side sounds good but the mono is really dynamic.
Unfortunately, the only vinyl pressing mistakes in my collection involve recordings that never should have been pressed at all, like all those digitally recorded LP's from the 1980's.

Anybody think they're worth anything? ;)
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I have a mis-labeled Arista Grateful Dead - Histroic Dead album with an Alan Parson's Project label on sides 3/4 of the album (vinyl pressing is still GD). My Goldmine's book doesn't recognize the error, but I still think it is worth something extra.