Actually, in KDE, rename doesn't give many errors at all, not for permission issues, if the file doesn't exist, or if your renaming a file to the same name. For the why question if your doing this in a shell, esp. one run by nightly/daily operators, you don't want errors like "couldn't rename so and so" displaying if these files are only created on certain days. If you want an error code use mv or add some manual checking to your script.