bad 4chan link
A post whose source is a direct link to a 4chan image, rather than to the 4chan post containing the image. These are considered bad links because there's no way to find the original 4chan post from the image link.
If you see this warning while uploading, it means you should paste the link to the 4chan post on the upload page, not the image itself.
Posts missing this tag can be found using this search: source:*i.4cdn.org/ -bad_4chan_link | source:*is*.4c*n.org* -bad_4chan_link | source:*images.4chan.org* -bad_4chan_link
See also
This tag implicates bad_link (learn more).
