Some built-in vars have special behavior that will coerce invalid values to valid ones. For example, /tg/ has this code:

dir = ndir
switch(dir)
    if(NORTH)
        offset_old = pixel_y
        pixel_y = APC_PIXEL_OFFSET
    if(SOUTH)
        offset_old = pixel_y
        pixel_y = -APC_PIXEL_OFFSET
    if(EAST)
        offset_old = pixel_x
        pixel_x = APC_PIXEL_OFFSET
    if(WEST)
        offset_old = pixel_x
        pixel_x = -APC_PIXEL_OFFSET

If ndir is null, this will end up setting dir to SOUTH. Our peephole optimizations incorrectly assume this will set dir to null and that we can use the value on the top of the stack instead of re-getting the dir value. This ends up doing switch(null) instead of switch(SOUTH).