I had to run some routine security updates for a client yesterday on a host OS running Hyper-V. It was after hours, so I just shut down the two virtual machines and rebooted the physical server after the udpates. When it came back up, I went into Hyper-V Manager and tried to power on my VM's. Low and behold, where there were once two VM's, I now only saw one.
I tried and failed to create a new VM and import the VHD to it, receiving a cryptic unspecified error message with some information that seemed to suggested a network adapter issue. I deleted the virtual network adapter and tried to create a new one, but that only resulted in further error messages.
Turns out Trend Micro decided I was most definitely a threat to the system and was destroying my virtual network adapters and crashing Hyper-V processes left and right. Awesome.