Only some users see forbidden error

Hi i have this really strange error that only a select few clients see. And they all get the error on the same page when posting from a form to another page. 90% of users seems to go thru fine. and the 404 page exists. Once they get the forbidden error they are unable to ever get past this form. i am really confused i would have thought is one person sees a 403 everyone would :/ un fortunately i can't do any testing as i dont get the error myself. any ideas ?
-- 403 error ---
You don't have permission to access /this_file.php on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Apache/1.3.42 Server at www.example.com.au Port 80

Do you have mod_security installed?

Also, check the error logs.

hi yes mod_security is enabled. ive gone through the error logs not really sure what im looking for nothing really stands out.

Well, the error log is a list of errors, so you're looking for bad errors. Or really any error at all. Even if they don't seem related to this problem they're still errors.

mod_security will reject requests if they don't met some good requirements or do meet some bad requirements. If the module doesn't have its own log file you can look through then there should be something in the site and/or global error logs.

thanks, i can see the user getting the 403 repeatedly when posting into the problem file. amongst this other people are using it fine. doesnt seem to be any other user or server errors surrounding this one. :/