In mod/forum/discuss.php there are some hardcoded error strings, and the first one (line 48) has an undefined variable (discussionid).

The users sometimes see this error : for example when they received a notification of a new forum post, the post is deleted but they still have the notification. If they click on it, they arrive at that page with that error. It seams a little too technical for them, and not in their language.

For that reason, it would be better if the string was not hardcoded, and could be translated.