Patron has a bib-level hold on a book. Between the hold being found and put in transit from another branch and being received to be marked waiting, patron finds a copy on their nearby branch's shelf and checks it out. It's not damaged/withdrawn/notforloan, hold isn't suspended, none of the known things that make holds unfillable. Now patron has one item from the record checked out AND they eventually have a waiting hold as well. While the found status does attach an itemnumber to the hold, if the hold was title-level to begin with, checking out an item from the reserve's bibliographic record should fill that hold.