That's not a bug, that's a negative light. Some people use these to "suck" light from an image and make shadows deeper. You'll find Sydd uses them a lot as do some other artists. Just select the thing and click on the trash can icon to get rid of it if you don't want it there. The negative lights are one of the few lights that Poser leaves behind even after resetting the lights, annoying but not world shattering. :)