Comment 10 for bug 1446760

I believe I hit this today in 15.10 (testing in VirtualBox). I installed kbuntu-desktop, and it asked which display manager I wanted to use. I just wanted to use the existing one, which I assume was lightdm. So I selected lightdm, and it set up sddm anyway. On boot up, the login screen freaked out, constantly flickering between lightdm and sddm. Both responded to password input, and when I launched Cinnamon, there were two sessions loaded on the same screen at the same time, two panels, one desktop session obscuring the other in a “picture-in-picture” fashion. The mouse did not register properly, there was massive graphical distortion, the system was unusable, and it completely froze after a minute or two until I finally managed to disable one of the DMs.

IMO, this is pretty severe. A non-technical user would be absolutely hopeless if this happened to them.