"Error: The operation completed successfully."
"Fatal error code 0: No error."

I once hacked a friend's command.com to replace the text "Bad command or file name" with "Out of cheese error".

Not really an error message, but when Microsoft first included virus scanning software in DOS, I saw the following:

The diskette in drive A: has a boot sector virus. You should reboot your machine immediately.

Note that it didn't tell me to remove the disk before rebooting...