ErrorDocument 404 problem

[FalseDawn]

I have the following line in my .htaccess file:
ErrorDocument 404 [mydomain.com...]

In my scripts, I have an include file, and I'm writing the contents of $_SERVER['PHP_SELF'] to a database and I notice that for every page
request, 404page.php is being written to the database.

I am using rewrites in my .htaccess file.

Can anybody who knows more than I do about the workings of apache rewrites and the $_SERVER['PHP_SELF'] variable explain why this is happening? Are the rewrites causing this page to be processed somehow, since I guess the page doesn't actually exist until it is transformed?

Thanks.

[FalseDawn]

Don't worry - I found the problem.
Didn't realise that missing images would cause this.
I had a bad file reference to a transparent gif of all things, so didn't spot it straight away.