When Mulder and Scully enter the abandoned mine through a doorway, the viewer can see at right in the distance a number of people sitting on a wall, and one crouching in front of the wall with what looks like a dog. They are sitting still, but there is still movement there.
Scully's father and the man Mulder thought was his father were both named William. Mulder's actual father, Cancer Man, was played by William B Davis. Scully and Mulder's son was also named William.