Scary moment occurred last week when one of my virtual machines failed to start from VMWare Workstation. The reported error was "failed to lock the file". A bit of searching revealed that I needed to delete the .LCK file in the VM's folder.
The cause for this (I think) was that I had opened the virtual disk of this VM as a mapped drive on the host machine (TIP: very useful feature to get at files from the VM without powering it up - just right click on the .VMDK file in Windows Explorere, and you'll see a couple of options there to map the disk as a drive). That drive was still mapped when trying to start the VM, meaning that the host machine was accessing the VM's files.