I remember when we did our own zmodem implementation so we could bypass download limits. Large BBS' at the time had restrictions on how many KB you could download in a day, but they only counted completed file transfers. ZModem had a provision for restarting failed transfers, so it was normal for a transfer to fail before completion (call waiting!) and the user to come back later to finish it. Our implementation could receive the last packet successfully, then push the transfer back a few blocks (retries) and fail.

Shout out to the 519!