Unrequited - S4-E16
Deliberate mistake: When Mulder is about to go into the metal detector he puts his gun on the counter. He goes through the metal detector on his cell phone but his keys set it off, when he steps through again it doesn't go off. No cell phones can go through metal detectors without setting it off.
Answer: As he stated many times throughout the series, Mulder needed Scully to be sober and skeptical. Whenever Scully's skepticism wavered and she started questioning her own rationality, Mulder would try to restore her sense of skepticism, because he needed her to be clear-thinking.
Charles Austin Miller