After remove Server.app, my apache2 on Mac OS X 10.9.2 Mavericks couldn’t start normally while everything looks OK. After carefully studied the log file through Console.app and looked into the content of /usr/sbin/apachectl, I realized that the starting process of apache in OS X is quite different than that in Linux: it uses launchctl!
So, I searched the apache launchctl config file:/System/Library/LaunchDaemons/org.apache.httpd.plist. Aha! it said:
The key SERVER_INSTALL_PATH_PREFIX and ProgramArguments were modified by Server.app aparently. Just recover them to /usr and /etc/apache2/http.conf, there, it works again!