KDE bug 275469 ( https://bugs.kde.org/show_bug.cgi?id=275469 ) is caused by Qt bug.
Steps to reproduce bug:
1) Close all applications and open Firefox (or any other app).
2) Close Firefox by clicking on the window manager close button (X), but make
sure your mouse doesn't move at all in the process. Firefox should close
cleanly without any ghost entries.
3) Now, start Firefox again.
4) Close Firefox using the same button, but try to move the mouse very quickly
to and from the button while pressing it. On my system, I get a ghost entry
almost every time I do this.
There are 3 patches which are fixing the bug. One can find them here:
http://bugs.kde.org/show_bug.cgi?id=275469 , comment #173. Please patch Qt for precise (and oneiric if it is possible) to workaround this annoying bug.