Okay, true story. When I was a sysadmin, we bought a backup product and backed up our windows servers. This was in ... 1997? One day we needed to restore some files. At that point we learned that the product only allowed restores if the hardware, filesystem config, OS, service pack, app, and app patch level were all *identical* to system at time of backup.

"But we backed up six months ago; we couldn't possibly replicate that."
Them: "No."
"So you wrote a tool that backs up but doesn't restore."
Them: "...Yes."