Sometimes, when logging in after rebooting or when switching between users, after typing the password and confirming, I get stuck at a purple screen. Sometimes I can escape it with ctrl+alt+F3 followed by ctrl+alt+F2, and some other times those combinations don't even work.
I understand that this is a bug in Ubuntu, but I'd like to know if there is a way for a user to work around this bug in the meantime.
UPDATE. After installing the proprietary drivers of the NVIDIA graphic card it did not happened again! Hope this helps! :)