Thanks AngelaS. I supposed that it wasn't possible to change the guest customization before it completes, but I wasn't sure.

Our case is as follows:

• A client applies for a new virtual server

• We have a group of preinstalled virtual machines that only needs the final configuration of the guest OS. So we take one of those preinstalled machines and customize it with the client preferred configuration.

• After this, we do a lot of external actions (not related with vsphere). If one of these actions fails, the process ends up with an error status and the virtual machine is not eventually assigned to that client, so it can be selected for another client.

If another client applies for a new virtual machine and the previous virtual machine is selected, an error arises saying that the virtual machine has a customizatin pending. So, as it's not possible to remove the pending customization, this virtual machine has to be removed, and a new virtual machine has to be selected to be assigned to that client.