Thank you for raising this. There has recently been a fix for the problem you describe, so it seems we have missed a way that this can happen.

To help investigate what needs to be fixed, do you remember any details about where the comment was being posted? For example, do you know which post or comment thread you were trying to post a comment in? If not, it would still be helpful to know whether it was a question, answer, or comment thread.

Yes, I have commented this question. The comment was long, and afaik it had also some formatting markups. Note I am using a firefox on Linux with a lot extensions, although javascript instability happens rarely by me, it is a possible problem source. I might try to reproduce it. But I think it is not a client side problem. I have also clicked "start a new comment thread". Nothing warned me that I should not.