B.7 Debugging Rewrites That Result in "Not Found" Errors

If your RewriteRule directives keep resulting in 404 Not Found error pages, add the PT (PassThrough) flag to the RewriteRule line. Without this flag, Apache won't process a lot of other factors that might apply, such as Alias settings.

You can verify that this is the cause of your problem by cranking the mod_rewrite logging level up to 9 and seeing that the entries relating to the RewriteRule mention something about prefixes with document_root:

RewriteLog logs/rewrite-log
RewriteLogLevel 9

% tail logs/rewrite_log 
ip-address  - - [date ] [reqid ] (2) prefixed with document_root to /usr/local/apache/htdocs/robots.text
ip-address  - - [date ] [reqid ] (1) go-ahead with /usr/local/apache/htdocs/robots.text [OK]

Don't forget to turn off the RewriteLog directive, or possibly just turn down the logging level, after you've done your checking! Otherwise your disk space may disappear like the snows of yesteryear.

Without the PT flag, mod_rewrite assumes that any rewriting it does will be the last URL manipulation the server needs to do for the request. Since mod_rewrite directives are handled very early in request processing, this can mean that Alias, ScriptAlias, and other URL manipulations may not get executed. Specifying the flag tells mod_rewrite to not short-circuit processing, but let it continue as usual.