If you have an image that is quite big and it takes some time to deploy it, the connection to the vcenter might drop and then the whole deploy operation will fail, while leaving a vm instance already deployed.
https://github.com/QualiSystems/vCenterShell/blob/develop/package/cloudshell/cp/vcenter/vm/deploy.py#L178
after the ovftool deployed the vm, the code tries to get the folder object, but if the session is already disconnected, you'll get an error from that line.
The error that you'll see will complain about the folder could not be found, while the problem is the session.
At that point there is no rollback and the deployed vm will remain in vcenter, while CS is not aware of it to clean it up later.