I've noticed this with post #662769. You can see its thumbnail e.g. at takenoko_(flamingo_garden).
The thumbnail was broken on upload but the post got added to pool #81 and a day later the thumbnail appeared fine so I removed it. Then a few days later I noticed the thumbnail was broken again, so I re-added it to the pool, and since then it randomly appeared ok or broken while remaining in the pool.
I investigated a bit and found that it got fixed only on one of the servers to which danbooru.donmail.us resolves, the other still has the original broken thumbnail.
So I guess that means that whichever script fixes the broken thumbnails only fixes them on one server, but not the other.
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