Came across an interesting one last week using https on an Opencart installation. After implementing https I discovered that all the secure pages generated a warning in IE6: “This page contains both secure and insecure items”. I couldn’t work out why this was happening at all and ended up with a test page with virtually nothing on it still generating the error.
After some more messing about it became apparent that the error was being caused by the CSS3PIE code – specifically the use of a reference to url(about:blank) in the script. There’s more info here. The suggested solution is to replace a.backgroundImage="url(about:blank)" with a.backgroundImage="none" which seems to have done the trick for me. I was a little nervous about that replacement so have created a separate version of PIE.htc just for IE6, with no problems so far.
Posted here in case anyone else has the issue as it took me a while to find the solution.