Found a new one for you just this morning. My laptop was plugged into an external monitor and I walked away for a bit. I came back a couple hours later, closed the lid, unplugged the HDMI and power and carried it upstairs just like I would do with my Macs or this same computer booted into Windows.
Whoops! How stupid of me. This is Linux, I'm holding it wrong. Unplugging a screen from a sleeping Linux computer is not reasonable user behavior, so an unresponsive black screen it is.
Yeah, I probably could have restarted the UI service manually by hopping over to another shell but it's flat out faster to hard reboot. Linux is still a 3rd tier desktop experience.