It turns out, that `B` flag in `RewriteRule` (mentioned in https://secure.phabricator.com/book/phabricator/article/configuration_guide/) was only introduced in Apache 2.2.7 (see http://ask.ttwait.com/que/915949), but official documentation just says it's available since 2.2.0: http://httpd.apache.org/docs/2.2/rewrite/flags.html
Could you please mention that Apache version must be at least 2.2.7 for Phabricator to work?