Quickly testing dangerous things in VMs with Hyper-V's checkpoints

Microsoft's Hyper-V virtualization platform has a fantastic feature called checkpoints. At any time, you can choose Checkpoints from the Action menu to create a snapshot of the virtual machine's disk and state. (It doesn't make a full copy of the disk, it just starts journaling changes.) If you make a critical error in some procedure, you can right-click the checkpoint in the Checkpoints sector of the Hyper-V Manager and choose Revert. After a few seconds, the VM will be back exactly as it was when you made the checkpoint, other than needing to be unlocked (because you reconnected to your session).

Once you're done with the checkpoint, you can delete it from the Hyper-V Manager. After that's done, changes to the VHDs will no longer be journaled, and the space used for the other state is freed.