Eric Siebert has writen a very good and explaining guide on how to troubleshoot snapshots on ESX 3.x.

Virtualization administrators can use snapshots on VMware ESX to travel back in time and figure out what went wrong with their virtual machines (VMs). But what do you do when your snapshots start acting funny? In this tip, we’ll troubleshoot potential problems that may come up when using snapshots on ESX.

Read it here or read my local copy.