I'm working with the computer and suddenly a top-level window with a "sad computer" appears. The window has a single button "Close session". Nothing else.

There are other programs in the background. I can Alt+TAB them but this windows raises itself, and doesn't allow to switch to other windows.
So I xkilled it. As response, it closed the session, LOSING ALL UNSAVED WORK.

From https://en.wikipedia.org/wiki/Sad_Mac#Sad_Mac I *guess* it tries to provide some kind of "catastrophic session error" message.
I would have expected at least a message saying so (even better if it has/points to some kind of log), not an unexpected window taking over the screen as it if were a virus.

Even if there was no option to restore the WM other than restarting the complete session (which there probably was), it should have allowed me to gracefully close the open applications, saving the files when needed.


PS: As it didn't identify itself, product is a wild guess.