Reproduction steps are:
Login to Liferay as a user who has permissions to add/edit content.
Create some webcontent item, set permission for Guest user to comment it ("add discussion").
On any page, add AssetPublisher portlet that would display that item. Configure that AssetPublisher portlet to allow ratings and comments.
Logout and open the page with AssetPublisher mentioned in previous step.
In that AssetPublisher, open the item that you've added. You should be able to comment it (have comment link present).
Click add comment link.
Enter comment text, click "Reply as..." - a login dialog will appear.
Click "Guest", fill-in username, email address and captcha, click "save" - comment will be posted, success message will appear with proposition to activate or cancel account.
Cancel account activation.
Click add comment link.
Enter comment text, click "Reply as...".
At this point a login dialog will appear already showing success message, though it is unexpected.