My inbox shows negative one (-1) notifications.
Not sure how I got there, somewhere around clicking on the notification for the Chat room owners can now establish room guidelines post. Refreshing didn't fix it. Clicking "Mark all as read" made it disappear once, then it appeared again. After clicking "Mark all as read" a second time it seems gone for good.
Not sure if worth a deep investigation, if it only happened once. Maybe it's possible to store notifications as an unsigned data type instead. Then this would never happen. It looks like this:

