Just finished troubleshooting an issue with a false alarm being triggered in vCenter after upgrading to vCenter 5.1. We have a custom alarm defined that warns if a VM has a snapshot larger than 15GB and alerts if it’s larger than 25GB. After the upgrade, all VM’s were triggering that alert even though they did not have snapshots that large – even VM’s without any snapshots were triggering. Turns out this is a known issue with vCenter 5.1.0 (799731). The workaround is to set the warning threshold to something less than 15GB and the alert threshold to something less than 20GB (we used 14GB for warning and 19GB for alert).