Steps to perform to reproduce the bug:
1- Make something crash such that a hard-error msgbox appears. For example, add a __debugbreak(); call to the wmain function of the eventlog service (in base/services/eventlog/eventlog.c).
EDIT: The BSOD seems to happen only when you make eventlog crash that way. If something else crashes (like e.g. regedit when it closes), the harderror msgbox doesn't trigger the BSOD.
2- When the msgbox appears, try to move it with the mouse. You get the following crash, see attached file reactos_r73029_harderror_move_win32k_crash.txt.
NOTE: Window messages:
70 == WM_WINDOWPOSCHANGING.
134 == WM_NCACTIVATE.