The Kmenu sometimes appears not where it should but with a shift to the bottom,
covering the panel. 

This depends on the icon theme currently in use: it does not happen with
Crystal SVG theme but does with Slick and iKons icon themes for example. The
menu appears shifted for the first click on the K menu button after a change in
KDE configuration through KControl. The second time the button pushed the menu
appears not shifted.

To reproduce set Slick icon theme, change something in KControl and push the K
menu button.

This is a very old and annoying bug.

Bug report in Trinity:
https://bugs.trinitydesktop.org/show_bug.cgi?id=352