During the second verse of the song (after Janet and her dancers have walked outside of the subway building) You will notice that three dancers, one of them having green hair, walk off camera to our left. As the verse progresses they show the three on the left dancing. In an immediate scene afterward, if you look behind Janet, there is the lady with the green hair!(I'm sure they are all there, but she sticks out). Then in the next scene they show those same dancers walk around to be behind Janet. It's funny that they would want to walk to a place they were already at.

I think this is because the two scenes were shot separately and then intertwined, but that's no excuse.