When a release has no associated CAA images, but does have a URL relationship -based cover art thumbnail (for example from an Amazon link), the cover art thumbnail from the URL relationship is displayed on the release page, and clicking on the thumbnail leads directly to the URL of the thumbnail. When a release has an associated CAA image of type Front, the thumbnail shown is the one from the CAA, and clicking on it leads to the URL of the CAA cover image.

The bug is this: When a release has associated CAA images, but no CAA image of type Front, the thumbnail shown is the one from the URL relationship, but clicking on it leads to the (broken) URL http://coverartarchive.org/release/MBID/front instead of the URL of the displayed thumbnail.

Here's an example: http://musicbrainz.org/release/4a8f4577-caab-4640-a9e2-3a38cb17d409/cover-art

(I intentionally didn't mark the cover image as type Front, in order to demonstrate this bug.)