This looks like you're creating a mem leak.
  Author: jimtabor
 Date: Sun Nov 30 17:39:20 2008
 New Revision: 37783
 URL: 
http://svn.reactos.org/svn/reactos?rev=37783&view=rev
<http://svn.reactos.org/svn/reactos?rev=37783&view=rev>
 Log:
 - Fixed hooking bug for Firefox, a plus, new SEH found a bug!
 Modified:
     trunk/reactos/subsystems/win32/win32k/ntuser/callback.c
 Modified: trunk/reactos/subsystems/win32/win32k/ntuser/callback.c
 URL:
http://svn.reactos.org/svn/reactos/trunk/reactos/subsystems/win32/win32k/nt…
<http://svn.reactos.org/svn/reactos/trunk/reactos/subsystems/win32/win32k/ntuser/callback.c?rev=37783&r1=37782&r2=37783&view=diff>
 ==============================================================================
 --- trunk/reactos/subsystems/win32/win32k/ntuser/callback.c [iso-8859-1] (original)
 +++ trunk/reactos/subsystems/win32/win32k/ntuser/callback.c [iso-8859-1] Sun Nov 30
17:39:20 2008
 @@ -507,12 +507,12 @@
     }
     _SEH2_END;
 -   IntCbFreeMemory(Argument);
 -
     if (!NT_SUCCESS(Status))
     {
        return 0;
     }
 +
 +   if (Argument) IntCbFreeMemory(Argument);
     return Result;
  }