Backups are the only macroscopic system we commonly deal with that exhibits quantum mechanical effects. This is odd enough that I've spent some time getting tangled up in these observations.
Until you attempt a restore, a backup set is neither good nor bad, but a superposition of both. This is the superposition principle.
The peculiarity of the superposition principle is dramatically illustrated with the experiment of Schrödinger's backup. This is when you attempt to restore Schrödinger's pictures of his cat, and discover that the cat is not there.
In a startling corollary, if you use offsite vaulting, a second quantum variable is introduced, in that the backup set exists and does not exist simultaneously. A curious effect emerges upon applying the Hamiltonian operator. The operator shows that certain eigenvalues are always zero, revealing that prime numbered tapes greater than 5 in a set never exist.
Finally, the Heisenbackup principle says that the user of a system is entangled with the system itself. As a result, within 30 days of consciously deciding that you do not need to run a backup, you will experience a complete disk crash. Because you've just read this, your 30 days start now.
Sorry about that.