1990-91 Upper Deck hockey error???
 

Ok I am not sure this is in the right spot or not.But if it needs moving then PLMK.On Sunday I was at our very small local card show.
Only about 30 tables.This gentleman came up to me and ask me if I knew anything about these two cards. So he showed me two
cards from the 1990-91 Upper Deck hockey set. And I must say I had never seen these two cards before.Mind you I have not seen a
lot and I am certainly not someone that does a lot of research into things. But he had a base Wayne Gretzky card from the set that had
the Theo Flurry back and the Theo Flurry card with the Wayne Gretzky back.This is likely a very common error but because I do not know.
I thought I would ask some of the more experts that may know about this error. I have include a scan of the cards just so you can
see which cards I am talking about. Any information would be greatly appreciated. Thanks.

https://i.postimg.cc/d1T523Jx/z39.jpg

https://i.postimg.cc/FK6Pk3XS/z40.jpg

We call those wrong backs, normally because the wrong sheet was fed into the machine, or the sheet was fed in rotated 180 degrees by accident. Here is another from the set, the only one listed on COMC.

https://img.comc.com/i/Hockey/1990-9...&size=original
1990-91 Upper Deck - [Base] - Wrong Back #381.2 - Ray Bourque Front / Daniel Berthiaume Back (Misprint) [Good to VG‑EX]
Courtesy of COMC.com

Some grading companies will note the factory error, PSA only will if it's a recurring error like the 1990 Donruss Nolan Ryan or the 1989 Heath/Brookens cards.
https://www.comc.com/Cards,sr,=1989+fleer+heath