When using Confluence, and Tomcat 5.5.26 or Tomcat 6 some users may find that their logins are not remembered. This is because of a bug in Tomcat's cookie handling. This is logged against the Atlassian Seraph library used by Confluence as SER-117. SER-117 has been fixed, so we "just" need to use a later version of Seraph.
Note that this only affects Tomcat users; Resin and Orion are not appear to be affected.