When Mulder first confronts Rob Roberts at his home, and there's blood oozing out of the garbage bag, there's more blood in the first shot than there is in the second after it's been leaking for a while.
When Rob removes his human disguise while hiding in Derwood Spinks' closet and emerges from it to kill him, the shot of his eyes when Spinks opens the door shows Rob with eyelashes. The eyelashes are part of his disguise-his character is hairless-but at no point does he remove them, as he emerges from the closet hairless.
When Robert examines his teeth in the mirror, the reflection of a crew member can be seen.
It is never revealed how Rob, a mere fast-food worker, can afford the numerous very expensive medical appliances he has to alter his appearance. The false ears, false teeth, full coverage contact lens, and other items would cost tens of thousands of dollars combined. Nor does it explain how he got them. They must be made by different specialists, and his true appearance would have certainly aroused suspicion among the various medical technologists that made them.
At 22:21, one of the employees of the burger joint can be seen looking directly into the camera as it pans around to follow Derwood.
Rob introduces himself at Overeaters Anonymous, "my name is Robert Roberts and..." People at Anonymous meetings customarily tell each other their first names only, not their last names.