Well this is very interesting! I managed to replicate it; when the key to an exit is set to 0, that's when the crash occurs. Here is another strange thing that's going on though; when you open or close a door, this is what you see on the other side.
This notifying the other room is found in the do_doorcmd function in act.movement.c, at the end.
This just might be a problem with CYGWIN though. I remember trying to use Vatiken's crafting snippet and it not working for some strange reason (even though the same thing worked for everyone else). It's making me seriously consider ditching CYGWIN for something else, though I don't have a free box to try and install (and learn) linux atm....
Edit: a combo of Windows 7 and CYGWIN actually, since I remember the former having issues with running older problems.
Also, looking at the entire function, it looks like part of the problem is that the pointer *back is initialized at NULL but is then never defined. Maybe I was being too harsh on windows and cywgin. Hehe (you can tell I edited this post a couple times). I'll take another look sometime tomorrow.
Code:
static void do_doorcmd(struct char_data *ch, struct obj_data *obj, int door, int scmd)
{
char buf[MAX_STRING_LENGTH];
size_t len;
room_rnum other_room = NOWHERE;
struct room_direction_data *back = NULL;
if (!door_mtrigger(ch, scmd, door))
return;
if (!door_wtrigger(ch, scmd, door))
return;
len = snprintf(buf, sizeof(buf), "$n %ss ", cmd_door[scmd]);
if (!obj && ((other_room = EXIT(ch, door)->to_room) != NOWHERE))
if ((back = world[other_room].dir_option[rev_dir[door]]) != NULL)
if (back->to_room != IN_ROOM(ch))
back = NULL;
switch (scmd) {
case SCMD_OPEN:
OPEN_DOOR(IN_ROOM(ch), obj, door);
if (back)
OPEN_DOOR(other_room, obj, rev_dir[door]);
send_to_char(ch, "%s", CONFIG_OK);
break;
case SCMD_CLOSE:
CLOSE_DOOR(IN_ROOM(ch), obj, door);
if (back)
CLOSE_DOOR(other_room, obj, rev_dir[door]);
send_to_char(ch, "%s", CONFIG_OK);
break;
case SCMD_LOCK:
LOCK_DOOR(IN_ROOM(ch), obj, door);
if (back)
LOCK_DOOR(other_room, obj, rev_dir[door]);
send_to_char(ch, "*Click*\r\n");
break;
case SCMD_UNLOCK:
UNLOCK_DOOR(IN_ROOM(ch), obj, door);
if (back)
UNLOCK_DOOR(other_room, obj, rev_dir[door]);
send_to_char(ch, "*Click*\r\n");
break;
case SCMD_PICK:
TOGGLE_LOCK(IN_ROOM(ch), obj, door);
if (back)
TOGGLE_LOCK(other_room, obj, rev_dir[door]);
send_to_char(ch, "The lock quickly yields to your skills.\r\n");
len = strlcpy(buf, "$n skillfully picks the lock on ", sizeof(buf));
break;
}
/* Notify the room. */
if (len < sizeof(buf))
snprintf(buf + len, sizeof(buf) - len, "%s%s.",
obj ? "" : "the ", obj ? "$p" : EXIT(ch, door)->keyword ? "$F" : "door");
if (!obj || IN_ROOM(obj) != NOWHERE)
act(buf, FALSE, ch, obj, obj ? 0 : EXIT(ch, door)->keyword, TO_ROOM);
/* Notify the other room */
if (back && (scmd == SCMD_OPEN || scmd == SCMD_CLOSE))
send_to_room(EXIT(ch, door)->to_room, "The %s is %s%s from the other side.\r\n",
back->keyword ? fname(back->keyword) : "door", cmd_door[scmd],
scmd == SCMD_CLOSE ? "d" : "ed");
}
more chance of human error and C) more difficult to accurately debug. 
