AlexisB said: Well, take a look at the pool description:
Why don't you read more than just the first sentence, Alexis?
Note that a thumbnail is not necessarily "missing" just because it is blank or odd looking: transparent backgrounds display as black, sometimes resulting in solid black thumbnails.
Danbooru fills in transparencies with black in thumbs and samples. This is no glitch; this is the thumbnail it's supposed to have. That's why it doesn't belong in the pool.
I read the whole description. Sorry for being pendantic, but the thumbnail does not look like it should be and as far as I know there is no "Broken Thumbnail" pool.
That's what the Missing Thumbnail pool is. But this thumbnail is not broken. It was generated correctly. You will always get this same result so long as the system remains the same.