I was looking at this Reddit post and scrolled down to the recommended posts on the right to see an uncensored NSFW thumbnail associated with this other post on r/programming from 16 years ago linking to an article. The image seems to have no relation to the content of the post or the article that the post links to as far as I can tell. I've confirmed with some others that going to the first post I linked while not logged into Reddit consistently recommends the other post with the explicit thumbnail. I was not logged into Reddit at the time.
I reverse image searched the image on Google and only got this third Reddit post (also seemingly unrelated) with the same explicit thumbnail. In both of the posts with the thumbnail, the images link to the exact same file on the redditmedia.com CDN.
What could cause this mismatched thumbnail? CDN hashing overlap? I'd be interested in knowing. Seems like a problem especially if it serves the image uncensored when I'm not logged in.