When a network player connects to a server (whether or not headles does, I think, not matter), and is lacking the currently set map, then this user is offered to download it. The user downloads it, the checksum test takes place, and there is an informational message saying that the map was downloaded successfully.
If, however, while the download takes place, the server admin decides to switch to a different map, then the checksum test will compare the checkum of the downloaded map to the map the server admin just set, which is different, and obviously has a different checksum. So in this case a checksum verification error occurs even though the downloaded map is probably fine.
This is a long standing bug, and I think it doesn't just affect maps but all kinds of mods.