Deleting an earlier comment on the same post while composing a new one causes the new comment composition to be canceled and the text being entered to be irretrievably lost.
As a secondary element of this bug, the add comment link also disappears after a user deletes one comment while attempting to compose another. The page must be reloaded to show it again. (Strictly speaking, it is not recreated. The add comment link quite properly disappears when the comment creation text box comes up, because a user is not intended to edit two new comments at the same time.)
I tested this with Google Chrome 28.0.1500.72 m on Windows 8 64-bit, and on Firefox 22.0 on Ubuntu 12.10 32-bit (provided by the Ubuntu-supplied package
firefox 22.0_build2-0ubuntu0.12.10.2). Chrome on Windows had AutoReviewComments installed; Firefox on Ubuntu did not. This suggests that platform, browser, and whether or not the pro-forma comment user-script is installed most likely do not affect whether or not this bug occurs or how it manifests (though I only tested those two combinations).
While still composing a new comment, I am poised to delete an old comment of mine on the same post:
I go ahead and delete the old comment:
Now the comment I was trying to compose is gone too (and secondarily, the add comment link is missing):
What Should Happen Instead
The expected behavior would be for the textbox representing the comment currently being composed to be unaffected by the deletion of an earlier comment, except perhaps that it should move up on the page to maintain vertical alignment (just as a posted comment would move up).
Fixing that would automatically fix the secondary bug where the add comment link is not recreated, because that link is not needed when the textbox for composing a new comment is still shown.
Rationale and Discussion
Since comments, when used correctly, are typically disposable, this is not likely to cause major data loss, but it is annoying, decreases productivity, and constitutes unnecessary unexpected behavior.
Furthermore, we encourage people to remove old comments that are obsolete (or which, on reflection, don't contribute very much). Often a user will, or should, remove old comments when posting new ones. The current behavior effectively penalizes this usually responsible action, and makes it unnecessarily difficult to use the system properly.
(This is not to say that users who prefer to remove old comments before or after, but not during, the composition of a new comment on the same post are doing anything wrong.)
Therefore, while the impact of this bug is not major, a fix would be beneficial.
Finally, note that the secondary bug of the add comment link not being recreated indicates that it's very unlikely the current behavior was intended.