un-backed up VMs

In your environments do you have any VM's that you do not backup as a matter of course? Our admins have sent through 'what we currently backup' in Veeam but a quick cross match to a list of all servers in the domain show some servers are not being backed up at all (which may be purposeful - hopefully!). I was just interested if this is common and examples of VM's where you don't need to backup the configuration/data?

Also in your experience - how easy would it be for an admin to miss something critical in terms of VM/virtual HD in backup selections? Realistic or very rare?