if a node has a populated image field (which does not have a default image specified) with a file that fails to load then the default image formatter will try to display a broken image.
on display
* image_field_load() calls file_field_load()
* file_field_load() tries to load all the referenced files, and replaces the entries which it did not find in the $items array with a NULL
* image_field_formatter_view() walks over all the $items and sends them to theme regardless if the content is NULL. with this, image derivative generation will try to show an image with an uri that doesnt exist.