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.
