OK, this must be one of the most serious Unity bugs out there as it renders the GUI totally crippled. I've been trying to understand its mechanism and initially I thought it happened only with Libreoffice but no. Do the following to reproduce:
1) Start an application that may use multiple windows and open a few windows (multiple pdfs in Evince, multiple Libreoffice documents, multiple Nautilus or Firefox windows, etc., anything). Even better, start multiple of those applications.
2) Now the launcher icon indicates multiple windows and you can click on it multiple times (without letting the launcher hide) to see different phases for window selection/management. First click to bring the most recent window to front, second to show all windows so that you pick one, third to hide them all. I believe I got it right so far, right?
3) Now try double-clicking or triple-clicking on the launcher icon, really fast. Then, pause briefly and do it again or do the same to another multiple-windowed launcher icon. In my experience, after a few double-click/window-select actions, what happens is that the Unity window decorator suddenly crashes. The thing is it crashes so bad that not only all window decorations (borders and buttons therein), the launcher and the top panel are gone, the GUI becomes totally irresponsive to keyboard input. Mouse works but doesn't help you get anywhere as you cannot shift through open applications or type a command to restart Unity. Ctrl-Alt-Del does not work but somehow Ctrl-Alt-FX (X is in 1-9) works and you can do "sudo reboot". Anything unsaved is gone forever...
This happened to me quite often and it did not improve after so many Unity updates/bugfixes. I think it is very critical and it has to be resolved for Unity to be deployed in a production environment. I wish I could get a screenshot but if you can imagine from the above description, it's pratically impossible to get a screenshot from the system.
I searched for a duplicate bug but I couldn't find. Thank you in advance for taking care of this!