Tonight, I tried copying a Parallels 6 virtual machine from one machine to another. I was greeted with the message, "The virtual machine cannot be used because its files are corrupted." OK, maybe there was a problem with copying over the network. I copied again, and received the same error. There is no option to actually try to recover the VM; the only options are to "remove" or cancel. There is an easy fix: you simply have to remove the virtual machine from your list of VMs and re-add it. The easiest way to do this is to click "Remove" when the error message presents itself and double-click on the VM again. Parallels then asks you (correctly) whether the VM was moved or copied, and starts up without errors.
Nice error handling, guys.