I see an answer (sorry, 10K only if it is deleted) that is full of only question marks:
How did that make it past the post filter? Is it a bug in that undisclosed algorithm?
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It didn't. The quality score on that was far, far below the threshold for blocking low-quality answers - as Anna notes, he could have just clicked through the warning anyway, but it doesn't appear that he even saw it...
Because the author of that answer didn't post that answer. He posted a different answer. I don't know what it contained, but it did make it past the quality filter.
And then he edited that answer, twice, within the 5-minute grace period. So there's no visible indication that the answer was edited, but whatever it originally contained was replaced with those question marks.
Why? I have no idea.