Interesting note: "one sentence in this paper is ambiguous [...] I am not going to remove this ambiguity or reveal where it is."

I find it surprising that he didn't actually attempt to change this problematic sentence if it was clearly shown that it leads to incorrect implementations. I get his point about prose, but a tiny footnote on a webpage doesn't feel like the best way to highlight what seems to be a common misunderstanding.

Any idea what this ambiguous sentence might be, and what is this mistake that implementers tend to make?