A post with a revision at the source that is bad, and should not be uploaded.
There's a number of more specific tags that can also be used:
Note that this tag is supposed to be used on image that are already tagged with md5 mismatch, not on revisions that are actually already uploaded. Images that use this tag should also have some amount of visual identity or sameness to the revision at source, and this should not be tagged on images that have actual revisions even if they are smaller resolution (post #2561932, post #2749109) or deficient in other respects. In other words, the deficiency should be the only change between the image on Danbooru and the revision at the source.
See md5 mismatch for more information on uploading revisions.