Basic Recommendations for Optimized Configurations and Limits for User Management in Confluence 6

• Avoid multiple usernames across directories: If you connect to more than one directory server, we recommend that you make sure your username is unique across the directory server. For example: We do not recommend that you define jsmith a . The reason for this requirement is to avoid confusion in the system, especially if you switch directory servers. Modifying the order in which the directory servers are sorted also modifies the order in which usernames are referenced.

• Be careful when deleting users in remote directories. When you connect using an LDAP directory, Crowd directory or Jira directory, be careful if you want to delete users in the remote directory. If you delete a directory that has data in Confluence, it will cause problems in Confluence.
• Avoid #(hash), /(slash) and ? in username (question) notation. There are known issues with using the symbols #, ? or/ for usernames in Confluence systems, as usernames using these symbols will not create spaces on the system. Please refer to https://jira.atlassian.com/browse/CONFSERVER-43494 and  https://jira.atlassian.com/browse/CONFSERVER-13479 for more information.

https://www.cwiki.us/display/CONFLUENCEWIKI/User+Management+Limitations+and+Recommendations