I am using the beta feature Discussion Tool, and this happened yesterday:
It is hard to describe any detail: I clicked on the Reply button, I typed the text into the Visual version of the tool, then clicked on the Reply (actually Save or Publish) button. The error message appeared instead of saving the message.
Reloading the whole page and clicking again on the Reply button solved the issue (and as a positive experience, the tool recovered the typed in text, which was an earlier request.)