I just had a customer point this out to me. Fortunately, it was someone I know, so they called me.

I've had problems before with the "Do you want to display the unsecure items" message, but I made changes and those are (mostly) gone. Now they told me (and I confirmed) that you get it when you get to the "Checkout Success" page. How can I figure out what isn't SSL on that page that should be?