Steps to reproduce this error:
Type something that will notify two users into the comment textbox. E.g
@user @user2 Hello!
You will see an error message with
Only one additional @user can be notified; the post owner will always be notified
Don't close the popup. Now, change the text you typed into a valid one, E.g:
@user Hello! Hi
- Submit the comment.
- Press the "add a comment" text again.
Result: Popup still resides.