Recently I had a reason to cancel a Deja Dup backup an hour later after it started. Now I wonder whether it messed up anything on the backup destination (AS3 storage)?
The worst that will happen is that some extra backup files will sit in the backup location until your next backup.
There are three ways a backup can end prematurely. Cancel, Resume Later, or being interrupted (like shutdown or loss of Internet). In the first case, Deja Dup will clean up any partial backup it might have left before closing. In the latter two cases, the files will be left, and it will try to resume the backup where it left off the next time it is run.