With fancy URLs turned on (rewrite), if a page doesn't exist PunBB sets "HTTP/1.1 503 Service Temporarily Unavailable" code. This may lead to some problems, for example Google won't verify the site, if it doesn't treat non-existent pages properly.
I examined the code - in rewrite.php the proper 404 code is set, but then the error() function is called and it overwrites it with 503.