I've been through this so many times with backup tapes that don't work, I'm beginning to see it as the norm. Unless of course you have a real backup procedure in place. Tapes have a fixed lifetime, they wear out. Because last nights' backup failed the verify. Chuck the tape and don't reuse any previous backups until you get another successful backup.
Drives get dirty.
Well, some newer DATs have a blinky light, but if your verify fails and the tape is fairly new, that's a good sign. To check it out, clean the drive, set the tape aside for the day after tomorrow, do the next backup on a new tape, then retry the tape that had the unsuccesful verify. If it fails again, its bad, chuck it. Drives break but don't tell you. When you need to replace the hard drive and find that all 7 backup tapes are corrupt, that's how.