A vm was displaying the warning that 'Virtual machine disks consolidation is needed' which is a nice feature of vSphere 5 which now actively tells you about this issue (It's always been there in previous releases but never highlighted in this way until 5.0).
We often get this issue as we use a snapshot backup technology to backup our vms each day and for some reason or other sometimes the remove snapshot process does not complete properly and we get this situation where the snapshots are removed but the snapshot files are still present and referenced in the vm. See the following VMware kb article for details.Consolidating snapshots in vSphere 5
Usually this is a simple process of right clicking the vm, selecting 'snapshot > consolidate' to have the snapshot child disk files consolidated back to the parent disk file but in this case the consolidation failed with the error message: 'A general system error occurred: I/O error accessing change tracking file'.
After some investigation I found that our backup system had a lock on one of the files and so I was able to release the file from the backup software and then re-run the consolidation which completed and all was good again!
The troubleshooting steps to identify the locked file can be found here: Investigating virtual machine file locks on ESX/ESXi
Previously I've also been able to resolve the issue of not being able to consolidate vm disks by creating a clone of the troubled vm and bringing it up as the active vm and then deleting the old one. Not always possible though in a production environment!