I am new-ish to SO, and I was creating a long comment. I didn't intend to, but the Answerer had written such a long Answer with many points.
I was happy with my composition and hit the Add Comment button... Nothing... So I hit it again, and for good measure a few more times... nothing. 'Is the SO broken today?' I wondered.
I copied my comment to a file so I would not loose it.
Much later (embarrassingly so), I noticed the 'characters left' indicator. 'Good. That means that I haven't reached some limit. I still have -1023 characters left'.
Then I thought 'what does the dash mean?' I typed a few more characters in and saw that it increased. - Ah! I have typed in 1023 characters too many.
Perhaps a red box across the screen to tell people that they have reached their limit?
But why allow extra characters to be entered anyway?