Excerpt from ITC BBS supporting this protocol:

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Date Posted     : 06 May 92
Subject         : QMODEM

It has come to our attention that several popular terminal programs,
particually Qmodem, does not properly implement zmodem.

If you already has the file you are abouts to download, Qmodem will attempt to
 resume transfer instead of skipping it, even thou you have the entire file.
This causes two things to happen.

1. The file will be corrupt.

2. You will be billed for receiving the file, even thou you already had it.
Resuming a transfer means you didn't complete it before and were not billed
for it.

If you attempts to upload a file that you did not tell the BBS about, the BBS
will demand a ZSKIP. Qmodem, and several others, do not support this part  of
Zmodem, even thou it is part of the specifications, and will LOCK-UP.