Caused https://bugs.kde.org/show_bug.cgi?id=339494
There's also a testcase posted that compiles w/ Qt4 & Qt5

The PID on the window is the pre-forked one.

NETWM spec states that:
"If set, this property MUST contain the process ID of the client owning this window."

Because:
"This MAY be used by the Window Manager to kill windows which do not respond to the _NET_WM_PING protocol."

Since the WM has no chance to figure that this is the wrong PID if there is a process with this (re-used) PID, this may also cause unwanted killing of the wrong process (while this will be a rare occasion)