We have a site that recently moved static theme images to Amazon S3 to offload some network traffic.
I put proper code in place to use HTTP or HTTPS depending on if they were on certain pages.
However, one particular page comes up as Mixed Mode SSL in Firefox and IE, Chrome reports it as secured. I don't see anything wrong with the page, all elements, CSS, JS and files are linked HTTPS to the source domain and to S3.
The only thing I notice is that 3-4 images come up as 302 Status Code when looking in the Chrome Resources inspector. Subsequent SSL pages come up as secured, and the only thing missing are those 3-4 images that are only on the storefront homepage. All other elements return status code 200, though I don't know what is different with these images, they work just like any of the others.