Not sure if this has been posted or not but at the gates of ahn'Qiraj if you keep walking into the wall (not the 'black wall') at a slight angle (so slight you can barely tell you're moving) you will eventually break through the other side. it took me a couple of tries but I can do it now in about 5 minutes. You'll see your self going deeper and deeper. Not sure if this works for any other place or walls that are invisible.