When connecting an external screen often windows are automatically moved to this screen. The screen is connected as secondary screen. I doubt that this is user expected behavior, at least for me it's not. Windows should stay on primary screen unless I explicitly move them to the external/secondary screen. This also happens when changing position of the external screen. It does not always happen to all windows. Reproducible: Sometimes
probably related to bug #317713 Please see comment #3 there and report (on whether windows don't actually change their position)
I did some experiments by attaching an external screen as secondary display. Not sure what exactly you need to know, so please ask if necessary. So in general it seems that windows keep their position relative to 0x0 (in the top left corner). This means adding/moving the external screen to Above or Left of primary screen all windows are "moved" there. Adding/moving it to Below or Right of primary screen lets windows stay on primary screen. The former case (adding to above/left) is unexpected behaviour. In both cases however there were individual windows which behaved in the opposite way. I didn't really see a pattern up to now however. E.g. my Kopete window was at the right screen edge of the primary screen. After adding the external monitor to the Right of the primary screen, most windows stayed on primary screen (as expected), but the Kopete window moved to the left edge of the secondary screen (y-coordinate remained the same).
*** This bug has been marked as a duplicate of bug 296673 ***