Any idea what a negative unread prayer request is supposed to mean?
My guess is that either a post was deleted or there is a race condition: that is, your screen refreshed in a narrow window between two counters being updated that caused a computation to be performed with one updated and one non-updated value.
Or maybe it means someone is praying for you.
In any case, I wouldn't worry about it. As a practical matter, the counter can never be assumed to be precisely accurate (not that there is any need for it to be).