system hangs if try end task with message box via taskmgr

1) open Notepad
2) Enter the text for the program to ask you to save before exiting.
3) Open taskmgr
4) Click "End task" for notepad

If you try move window with question, system "hangs" and\or taskmgr show painting problems.

UPD: The shell may be hanging because I was able to log off.