Clicking the "Maximize" button in xterm's title bar results in a maximized window, BUT KDE forgets immediately that it is now maximized; this means that clicking the same button again does not restore the window's initial size. Instead, it appears that the second click seems to inform KDE about the size change, because only then the small icon inside the button changes into the shape for maximized windows. Clicking the button a third time changes the icon back to the shape for normal windows, but as the size was at maximum already, it doesn't restore the initial size.

I found this only with xterm in KDE so far, other applications work properly, and even xterm works properly in FVWM, ICEWM and Gnome. So it appears to be a bug in KDE.