I really have no clue, but what I guess is the case, is that the bug appears depending on what you have done earlier. Maybe it is so that if you miss a side-area, this area doesn't load correctly. If the testers were crappy, they only went through everything each time, and thus couldn't see this.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS