1. Plug in an external screen 2. Mirror it to the internal Laptop screen using the OSD 3. Open the KScreen KCM in System Settings 4. Un-mirror it by dragging the screens apart from one another (and click "keep" in the "are you sure" popup) 5. Mirror again using the OSD Result: the KCM shows incorrect settings from before you mirrored; the screens are not on top of each other in the view, and if their resolutions were changed during mirroring (e.g. due to Bug 460246), the resolution combobox does not show the actual resolution. Closing the KCM and re-opening it reloads it and makes it show the correct state of reality.
I have also noticed repeatedly, that the screen arrangement in Display Configuration is not saved correctly, after disabling a screen (by selecting it and unticking "Enabled") and then re-enabling it later, after restarting the X11 session. However I still can't reproduce this a 100%.