I suppose you could consider it a bug. It's the type of thing that happens a lot in games, though not as much now as it did in Nintendo 64 and Playstation games. I don't know much about coding, but my guess is that the developers simly didn't make an object un-passable. For instance, in the old Mario Kart 64, if you rammed one cart into another, you could knock it through the wall. It's really quite a common problem in 3D titles, and not one to overanalyze.