The default behaviour in some systems (Rails, for example) is for 404 errors to use a controller to handle the behaviour. Which means your CPU spends time trying to process the request.. fails... then spends more time processing the error controller!
Here, then, is a collection of the smallest possible files which you can freely use as placeholders to enhance your sites performancs.
GIF - Transparent (43 bytes)
GIF - White (35 bytes)
PNG - Transparent (68 bytes)
PNG - White (67 bytes)
JPG - White (43 bytes)
MP3 - Empty (32 bytes)
SWF - Empty (12071 bytes) - To be improved!
TXT - X (1 byte)
null file - X (1 byte)
Note: The null files of 1 byte can be shorted by just touching the appropriate filename. However, some systems don't like storing them!?!?!