.whenever a downloader doesn't have something they currently would ask a peer for in unchoked, they must express lack of interest, despite being choked.
2.When a download is almost complete, there's a tendency for the last few pieces to all be downloaded off a single hosed modem line, taking a very long time. To make sure the last few pieces come in quickly, once requests for all pieces a given downloader doesn't have yet are currently pending, it sends requests for everything to everyone it's downloading from.
3.It's possible for an unexpected piece to arrive if choke and unchoke messages are sent in quick succession and/or transfer is going very slowly
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