The "Delete source file after copy" does
So far I've noticed this happens when using the copy command and the sftp operation but it probably happens on every operation that has the "Delete source file after copy" option available.
As VC doesn't have a native "Move files" operation, the way to accomplish this is using a "Copy files" operation and checking the "Delete source file after copy" option. However, when using native tools in Windows to move a file (using cmd, powershell, cut/paste, etc) Windows does not copy the file to the new location unless it can also delete the source file. So if a file is in use, it will not be moved to the new location.
Visualcron does not work this way however. If "Delete source file after copy" is checked and a source file is in use, it is still copied to the target location but then is not removed from the source location (because access to delete it is denied).
To counter this I reverted to not using the internal copy operation in VC but using VC to call a powershell script that does the move instead.
I feel VC should have a check built in if it can delete the file before it copies it, when the "Delete source file after copy" option is checked.