Unfinished download is a frequent issue in most peer-to-peer download architectures. For some networks, it seems to be an inherent problem, however even «smart» networks can encounter the whole file but one block downloaded case, which we call the missing block paradigm. We propose a simple and versatile model of file-sharing that applies to all block-oriented file-sharing protocols used in softwares such as MlDonkey or BitTorrent. Simulations using this model show that missing block can occur even with a popular file, and lead to some theoretical explanation. These fundamental results offer a new understanding of downloading issue in file-sharing networks, and show new strategies existing protocols could use.