After rebooting a machine using the lightdm display manager, garbage / parts of the user's previous session are displayed on the login screen, which is an important security problem, as anyone with physical access to the keyboard and screen of the machine can reboot it (no need to have an account for that). See Debian bugs:
The problem can also occur after logging out. And parts of the user's previous session can appear on the whole background screen. That's a lot of data!
Corrupt background also occurs after a cold boot (i.e. from power off), but in this case, there is no security problem of course: I just get a blank background, possibly with random black points.
After the latest Debian/unstable upgrade and reboot, in particular with the xserver-xorg-video-nouveau package upgraded from 1:1.0.10-1+b2 to 1:1.0.11-1, it seems that I can no longer reproduce this problem. I've tried several times after logging out, and only once after the reboot due to the upgrade.