This is one of those things you learn the hard way. The ability to snapshot a VM in VMWare is a great feature, but unfortunately if left unattended could grow to be a bit of an issue. Snapshots are great if you need to make changes and need to quickly undo them.
However once you take a snapshot, VMWare starts creating another file which is essentially a copy of all the changes that are happening since the snapshot. Once you are sure you are done with your work you should remove the snapshot instead of leaving it there. By leaving it there, the other file can grow quite large and affect the performance of your ESX server. Also if you finally do decide to remove the snapshot much later on, then the VMWare server will have to write all those changes back into the virtual machine. The more time that has passed, the more changes that need to be written. This can also slow down a server and I have even seen it crash ESX servers to the point where the virtual machines were still running but the ESX server itself couldn’t be managed at all from Virtual Center.
So the rule of thumb is to take a snapshot before rolling out a patch or update of some sort, do your testing and then remove the snapshot in a quick amount of time so it doesn’t have time to grow out of control.