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It happens for me a lot less after 10.13.2, but unplugging from a LG 4k or 5k, or sometimes opening the screen out of clamshell when still connected crashes the OS (yellow screen on laptop, red screen on LG).
It happened every single time on 10.13 and .1, and the only way around it was to log out before disconnecting or opening the lid.
What Mac model? and are those Thunderbolt 3 connections or some other way?
We have LG 4K's but a lower end model that is Displayport or HDMI connections with out any issue like that. (USB-C to HDMI direct from laptop worked fine but with the Elgato Thunderbolt 3 dock found that we had to use USB-C to Displayport to get things to play nicely - using the DisplayPort connection on the dock directly had weird detection issues - will be doing some testing with some HP 4K's to see if the same problem shows up - through the dock USB-C to HDMI resulted in a weird lag to the LG 4K that didn't happen when connected any other way)