0028a63 created the issue

Resuming large (few 100 MB) binary files in my experience often lead to invalid files uploaded. It happens that the resulting file on a server is a few hounded MB larger than original. While resuming the "uploaded" field in those cases show invalid (smaller) amount of transfered file.

For example, if a transfer was stopped at 400 MB of a 500 MB large file resuming (with "resume" button in Transfers window) starts for example at 300 MB. Obviously this is not just a bug in a transfer reporting but it really starts sending a file from that point. And the result is a 600 MB large file.

It seems that Cyberduck is not using proper value to calculate the resume point.

I saw in some other FTP clients that they first download a few tens KB of a file which they will resume to check if that matches the data in a file at the resume point. It is not fail proof concept but it is better than nothing and makes appending files (thus resuming FTP transfers) safer. It should be at least an option to do that.