Darkroom

The back cover of the various DVD and VHS releases as well as the film's trailer reveal the identity of the killer, thus spoiling any tension to be developed upon viewing.

When Janet is showering, the killer is taking photos of her through the window. At some point she hears the clicking of the camera. That would be impossible, not only due to the noise of the shower and washing her hair but also the humming she was doing and the fact that the window was closed.