Usually it wasn't the tapes that were unreliable, but the tape devices (firmware) and/or the drivers. Years back I used to work on a backup product - one of the tape devices would write a random amount of zeroes after each round of N megabytes written to the tape. So if you were "lucky" and all the backups were below N megabytes in size, everything worked perfectly. We weren't even aware there was an issue until some of our customers started complaining.

Debugging this was really fun, especially due to the randomness factor. :)