As discovered in CAA-129, there are a couple hundred items in CAA that don't have the noindex flag set, although IA requested that we do so. Also, there are roughly the same number of items that have their mediatype set to something other than image.

I've sampled a couple of those releases, and it turns out this is happening on merged releases. Take this item as an example. The cover art was added to a release which was later merged into the release to which the item belongs. The merge target at that point does not have an IA item, so moving them will create the item, and it seems that the correct headers aren't supplied when that happens.