In the event when you restore VMs from backup or when delete Snapshot operation fails, then Snapshot List and actual Snapshot (Delta VMDK files) could become inconsistent. In this case VMs require Snapshot Consolidation, otherwise Delta-VMDK file continue growing its size without your ability to delete the snapshot as it is not listed in the Snapshot Manager.
In vSphere 5.0 above, VMs that require Snapshot Consolidation can be shown the message as you see below.
And you can reconcile this by simply selecting Snapshot > Consolidate.
However, this will not come up as a error or warning message so unless you select a VM from vSphere Client you will not be able to discover if a VM requires Consolidation.
To discover which VMs require Snapshot Consolidation, you can use PowerCLI command.
The command below displays the list of VMs and see if Consolidation is required or not.
Get-VM | Select Name, @{N="Needs Consolidation";E {$_.ExtensionData.Runtime.consolidationNeeded}}
The Command below displays only VMs that require Consolidation.
Get-VM | where {$_.ExtensionData.Runtime.consolidationNeeded} | Select Name
I’ve learned this trick from the post on VMware Community forum.
http://communities.vmware.com/thread/345294?start=0&tstart=0