I don't have a "most painful error".

The real pain for me is the

WHY ISN'T THIS WORKING

I'VE DONE THIS 1000 TIMES BEFORE

THIS ISN'T HARD

THIS SHOULDN'T TAKE THIS LONG...

It's just the worst combo of events / feelings / leads to the hopeless depths of imposter syndrome and etc.

I feel hopeless after Unity fails to start/run under your primary user but works fine using guest user and other user in the system. And i did my research to fix it, but none seems to work. :(

The last thing i did to my laptop that might cause the error is executed the commands below:
sudo apt-get update
sudo apt-get upgrade

And that feeling when you google your error message and get 3 hits, none exact, 2 of them in Chinese and one automated malware scan results.
It really makes me feel that I'm pushing the industry forward.