After we rubbed the lamp, the Djinn appeared invisible for about 2ish minutes -- still able to attack us, but not attackable. I've run through Boss Rush about 15 times or so, pretty fun, and this is the only time it's happened. The good part is that he went back in another lamp and appeared visible again the next time around. You also only get 8 XP for beating the Djinn boss. Anything after Dragon is really low XP, not sure if that was intended.

Link to screenshot: http://cloud-2.steampowered.com/ugc/...A637153CCCF63/