Apache on Red Hat Linux with with the UserDir directive enabled generates different error codes when a username exists and there is no public_html directory and when the username does not exist, which could allow remote attackers to determine valid usernames on the server.
https://exchange.xforce.ibmcloud.com/vulnerabilities/7129
http://www.securityfocus.com/archive/1/213667
http://archives.neohapsis.com/archives/vuln-dev/2000-q3/0094.html
http://archives.neohapsis.com/archives/vuln-dev/2000-q3/0087.html
http://archives.neohapsis.com/archives/vuln-dev/2000-q3/0083.html