When using ESX VC Snapshot manager, it is a good idea not to leave your snapshot online for more than a day or two.

When a snapshot is left for more than 24-48 hours, disk "delta" files become very large and difficult to commit to the original Vmware vmdk disk file(s). This can lead to data corruption on your original vmdk file.

See the following links:
http://www.vmware.com/community/thread.jspa?messageID=584324&tstart=0
http://www.vmwarez.com/2006/11/beware-long-snapshot.html

* When you go to delete a Snapshot, via Snapshot Manager, you may see the process get to 95% and then seem to hang----then fail with the error: "Remove Snapshot: Operation Timed Out." DO NOT issue another delete via Snapshot manager.

There is a background process that is still removing the old snapshot. Be Patient. When the process is completed, you will NOT see the snapshot listed in Snapshot manager. The -delta.vmdk files will also be gone.

Unfortunately, I've experienced this first hand.

-Will