In IE 6, when viewing a project or issue page over https, you receive "This page contains both secure and nonsecure items"

This can be duplicated on a 4.0 instance of Jira (including, at the time of reporting this bug, support.atlassian.com)

An example page where this can be seen is:

• https://support.atlassian.com/browse/JSP

To duplicate this problem, under IE 6, set the following setting
Internet Options dialog -> Security tab -> Custom Level -> Miscellaneous -> Display Mixed Content to "prompt".

If you set it to "enable" you should not get the warning.