This problem only appears in one document (I think).

I enabled "Readability Statistics" while working on document X. Now, for document X:

1. The Spelling and Grammar check detects no spelling and grammar errors (and there are indeed errors);
2. While the "Readability Statistics" are indeed displayed, all fields are 0 except for "words", "characters", and "paragraphs". These numbers are all too low and, get this, they are not constant - they sometimes remain the same upon re-run of the test, and they sometimes change. But they are always wrong in the sense of being too low.
3. Word Count appears to function properly.