Visible crew/equipment: Near the beginning when the "cavemen" are looking around after hearing the noise from the alien, you can clearly see what looks like a sneaker print in the ice, in 35,000 BC. (00:03:00)
Continuity mistake: When Stevie the little boy is in the cave, there are basically two shots: One from behind his three friends looking down, and one from Stevie's perspective, looking up. Every time we look down into the cave there is quite a lot of sand trickling down, but when we are with Stevie in the cave looking up, there's no sign of it. (00:06:25)
Factual error: Dallas, Texas is not surrounded by desert mountains. This is a dumb tourist mistake; when that shot of downtown Dallas appeared on the screen audiences there groaned in unison. (00:07:30)
Continuity mistake: At the beginning where Mulder and Scully are on the roof searching for the bomb, you can clearly see Scully gripping the door knob and she holds it. When Mulder tries to grab the same door knob, Scully gets out of the way and crosses her arms. The next thing you see is Scully with her arms uncrossed. (00:14:13)
Visible crew/equipment: Near the entrance of the building, when the fire brigade arrives, a guy holding a boom microphone is visible. (00:19:25)
Continuity mistake: When examining the body of one of the four fallen firemen from the explosion, Scully is wearing a dark grey suit. In the close-up shot of her hand with rubber glove, the arm is wearing a white doctor's coat. (00:37:40)
Character mistake: Mulder and Scully sneak into the morgue at Bethesda Naval Hospital to view the body of the dead fireman from the bomb blast. Mulder lifts the sheet back on the body to show that the man's tissue is "like jelly." Scully surmises there's been some kind of cellular breakdown, and then says the tissue is "completely edematous." However, Dr. Scully mispronounces "edematous" as ed-a-may-shuss. The correct pronunciation is i-dem-a-tuss. Being a doctor she'd know the difference. (00:37:45)
Continuity mistake: In the Dallas lab, Mulder takes the test-tube that contains the fossils and give it to Scully. Then, he gives it to her again. (00:46:42)
Continuity mistake: As Mulder and Scully enter into the building that houses the bees is seems Mulder's tie gets blown over his right shoulder and then he is seen inside and it is over his left shoulder. (01:00:30)
Continuity mistake: When Mulder and Scully start frantically running from the bees inside the dome Scully is clearly seen untying her belt twice. (01:01:55)
Continuity mistake: In the bee dome, Mulder yield at Scully: "Give me your hand!" He grabs her wrist. The next shot his hand is touching her shoulder and drops in her back. The next shot, he is holding her hand. (01:04:30)
Continuity mistake: In Mulder's apartment hallway, when he and Scully are about to kiss, she let out one tear from her right eye. The next shot we see Scully, the trail of the tear has disappeared and also has the tear. (01:13:18)
Continuity mistake: When Mulder encounters the Well-Manicured Man, all doors of the limousine are closed. However, in the next shot the driver's door is open. (01:17:25)
Revealing mistake: When the booby-trapped car explodes, the detonating wire is visible running out of the exploding car and out of shot. (01:22:55)
Factual error: The stamp of the scene says that it is Wilkes Land, Antarctica. Wilkes Land extends from Cape Hordern in 100°31' E to Pourquoi Pas Point in 136°11' E. However, the coordinates of the spaceship are 63°00' E, 83°00' S, so this place is located definitely outside the area of Wilkes Land. (01:24:00)
Continuity mistake: While Mulder climbs down the inside of the spaceship, when he enters the second hole he wears gloves but when he falls out of if the gloves are gone. (01:27:20)
Factual error: When the Smoking Man comes back to the base, in the first shot the tracks are shown, but they definitely do not belong to his snowcat but Mulder's. Only Mulder's snowcat has triangular-like shape of the tracks. (01:33:05)
Continuity mistake: In the spaceship, Mulder unbuttons Scully's jacket to perform CPR on her. The coat is undone about 13 cm under her chin. Later, it is clear that the coat is buttoned up again, up to under her chin. (01:42:05)
Continuity mistake: When Mulder has Scully thrown over his shoulder and has to set her down to perform CPR when she stops breathing, her hair is pasted over the side of her face. Then, all of a sudden, the hair is gone. When Scully starts breathing again, Mulder smooths back her hair, and in the very next shot, the clump of hair is back. (01:42:30)
Revealing mistake: In the end when Mulder is reading the paper in the park. The paper is clearly a prop. Only the frontpage is real - when he puts the paper away you can see that the rest of the paper consists of blank pages. (01:45:45)
Chosen answer: If they had put an "Out of Order" sign, or left it unilluminated, someone in charge might try to have it fixed or replaced, or could have checked to see if it's plugged in, etc. By leaving it illuminated, it doesn't look out of place and simply not taking money wouldn't arouse too much suspicion. In my own life, when I've come across a seemingly working vending machine that won't take my money or dispense drinks (but gives my money back), I just find another one instead of calling someone or reporting it. But yes, it is very convenient to the plot that Mulder just happens to try and use this particular machine.