The main problem with Time Machine is that by default it takes snapshots every hour.
There are third party plugins which allows to change this behavior to once a day.
What is the problem.
Let say Time Machine is backing up Parallels Desktop
and example file order is
Snapshot.xml
VirtualMachine.pvs
--etc
winxp.hdd
There is some seconds between Snapshot.xml or DiskDescriptor.xml has been backed up, and to go to the winxp.hdd file and during those seconds you write some data to disk
After that you turn off your Mac, not waiting to the next hour data will be synchronized
And some how disk failed
If you restore you will not be able to start VM, as there is difference between data in Snapshot.xml and in real snapshot file, the same is correct for DiskDesciptor.xml file in winxp.hdd bundle
Microsoft Windows use Volume Shadow Copy for such situations, but Mac doesn't and this is the main problem we recommend to use our tools for backup purposes.
Please use one of suggested backup methods as described in Parallels Desktop for Mac user guide from
http://www.parallels.com/en/download/file/doc/Parallels_Desktop_for_Mac_User_Guide.pdf on page 243
