Currently rebuilding my student association's infrastructure, I have to deal with Proxmox 3.4 quit a lot. Having up to 5 VM, I decided to first install one, called generic, that I would backup, then restore 5 times to get all my VM with a basic configuration without efforts.
That worked perfectly a first time, and I have been able to deploy our Gitlab quit quickly.
Then I wanted to work on our proxy VM, so I restored the generic image, and logged into the VM with the console interface to change at least its name and IP. After that, I was able to ssh to the machine, but I remarked that when typing a command, it was really slow to answer.
Some quick ping based investigations told me that there were many lost packages, and the average ping was up to half a second! Of course, everything was in a local network, and pinging the Proxmox host was pretty normal, just as the first Gitlab VM. Why was that second machine causing so much trouble?
The answer was very dummy: restoring an image restores really everything, and even the MAC address! Changing that address fixed the problem, but that story reminds me how shitty I am with networks! :D