There was a fascinating bug in older versions of KVM on CentOS. Specifically, a KVM virtual machine had no awareness that it was not running on physical hardware. This meant that if the host OS put the VM into a suspended state, the virtualized system clock would retain the time that it had had when it was suspended. E.g. if the VM was suspended at 13:00 and then brought back to an active state two hours later (at 15:00), the system clock on the VM would still reflect a local time of 13:00. The result was that every time a KVM VM went idle, the host OS would put it into a suspended state and the VM’s system clock would start to drift away from reality, sometimes by a large margin depending on how long the VM had remained idle.