Turning all screens off in Plasma-workspace and kscreenlocker while the lock screen is shown can result in the screen being unlocked when turning a screen on again.
https://www.kde.org/info/security/advisory-20160209-1.txt
https://bugzilla.opensuse.org/show_bug.cgi?id=964548
https://bugs.kde.org/show_bug.cgi?id=358125
http://lists.fedoraproject.org/pipermail/package-announce/2016-February/177557.html
http://lists.fedoraproject.org/pipermail/package-announce/2016-February/177454.html