Oh, what fun! Our system administrator decided once to duplicate the boot partition of a server, so we had a spare should the usual one fail. He used dd of course. And the next day, he was off to a USENIX conference. What he had forgotten, though, was that the first partition on the disk actually contained the partition table! (This was on Solaris, or possibly SunOS.) Since the source disk and the target disk had different partition tables, things started falling apart. It did not happen all at once, probably due to caching. But we started getting more and more weird errors. It took me half the day to figure out what had happened. By great luck, I had actually saved a copy of all partition tables just a few days prior. They were in human readable form, but good enough for me to restore the damaged partition table to its original state. A reboot later, all problems were gone.