When I open a new comment field, but before I have entered anything, next to the Add Comment button it says "enter at least undefined characters". When entering something then deleting all of it, the same message appears. This occurred on the page Can I POP a value from the stack, but put it nowhere in NASM Assembly? but seems to happen regardless which page I use.
This is on Mozilla Firefox 68.0.2 on Android 9. No userscripts I'm aware of. By next to I mean that below the comment field, on the left hand side there is the Add Comment button, and in the middle (to the right of the button) there is the status message indicating either what I reported or (eg now) "267 characters left".