422 (Unprocessable Entity) Error when trying to create a comment on a PR

I pay for private repos, and this has always worked in the past, but now I’m trying to save a comment and start a review of a PR and I’m getting a 422 (Unprocessable Entity).

POST https://github.com/jholovacs/My.Project/pull/66/review_comment/create is failing.

I have cleared the cache.  I removed all cookies.  I relogged in.  I am still getting this error.  What gives.

OK, so I think the problem was I was looking at an outdated update.  I cancelled my review, started a new one, and it worked, though if it was an outdated post I was looking at, it shouldn’t have offered to let me create a comment to begin with.