red border
Posts with a red border around the image.
Posts flagged for deletion on Danbooru were in the past marked with a red border (rather than the blue one currently used, indistinguishable from the one for pending posts; approvers still retain the red border for flagged posts), hence they were formerly a subject of pool #2483 ("What Do You Mean Flagg... Oh").
See also
This tag implicates border (learn more).
