Don't you hate it when that happens. I've been to places where they thought floppies and/or zip drives were a reliable form of backup media and were amazed when about half of their disks wouldn't read. I've had tape and cdr and cd-rw failures as well as external hard disk failures. Most recently my USB flash drive failed and I got it exchanged under warranty (and I was impressed when Lexar sent the e-mail asking if I needed data recovery or not).
Also test restores if you can. I have backed up and verified peoples data and then later found out they saved documents in other places than where they told me (intentional or not) and no longer had them.