Another example would be if you have an install of Windows for example, and then you install Ubuntu with FDE. Just because you have used FDE when installing Ubuntu does not mean that the entire drive has been encrypted. Only the newly installed data related to Ubuntu has been encrypted, and only the space it takes up has been overwritten. Unless you first did a separate full drive overwrite, there will still be plaintext data remnants from the Windows install on the drive. The exact details of if you can count on an FDE implementation to be equal to a full drive wipe or not will vary from implementation to implementation. But having FDE is not the same as having wiped your drive, so you should not confuse the two things and rely on FDE to be a secure wipe.