Microsof snapshot (called so in Hyper-V manager) or checkpoint (called in SCVMM) functionality is a nice feature for protecting your server before applying hotfixes etc.
There is a nasty thing about snapshots: they are only deleted when the virtual machine is shut down, stopped or put in a save state. If you do not delete the snapshot and reboot the virtual machine you might run into problems.
Read for instance this article of Mikael Nystrom.
Eric Gray of vCritical.com has an opinion about this shortcoming!
Would this be fixed in R2:?