This came up while discussing post #146057. If you look at the full picture, the quality of the lines is rather poor, making it not up to standard for danbooru; however, you can't see that from the thumbnail alone. Whoever approved it probably only looked at the thumbnail.
There are many other possible flaws that don't show up in thumbnails, such as JPEG artifacts, bad scan quality, limited resolution, and so on. When in doubt, it's better to look at the actual image.
Updated by jxh2154