There was an interesting bug reported for Twenty Twelve recently: The .single-attachment body class only gets applied when the attachment’s parent is a post (or custom post type). If the attachment’s parent happens to be a page, that class is missing.

In case you have been using .single-attachment to specify styles for the image attachment page, make sure to test your theme with images that have a page as a parent. And use the .attachment body class in the future. ;)