I would have to say when objects are geomerged for no particular reason. It's ok if they're supposed to be something, then that's ok, but otherwise, forget it.
Also, about the corner wall thing, my friend, his step-bro, and I made an ok map with them on their side like ramps, and I don't think it's too bad. If you want to see the map, download from here:
Bungie.net : Bungie Online : Halo 3 File Share. BTW, it's from my friend's File Share but me(Lcarnacieg), Sudd3n Sh0ck, and Calamity Strike made it.