I passed some usb mass storage device through to my VM, I stopped the VM and removed the usb device. When I later tried to start the VM again it complained about a missing usb device.
Since USB has hot plug capabilities I would think that is better to give a warning instead of an error. But even worse I could not remove the usb device in the settings since it was not shown if not plugged. I was lucky that I still knew which usb stick it was so I could plug it in and disable the pass through in the settings.