Continuity: Doug and Tony are being interrogated by the Mars Mission flight crew. In two-shot, they're standing shoulder-to-shoulder with each other, no more than a few inches apart. When the shot cuts to a wider angle, Tony is suddenly several feet away.

Revealing: During the final fight scene, Doug is struck by one of the guards and falls back against a rock. The huge boulder wobbles precariously when he hits it.

Factual error: The aliens show Doug and Tony the planet they plan to invade - Earth, of course. But the special effects department forgot to give poor old Earth any clouds. (There are clouds over more than 70% of our planet at any given time.) When they go down to the surface, though, there are plenty of clouds in the sky.

Continuity: An escaping Tony leaves the cell door standing open. But when the shot reverses angles, the door has mysteriously closed itself.

Continuity: In the saloon, the table at the lower right of the shot has nothing on it. After a brief close-up of the bartender, we cut back to the master shot - and even though no one's gone near it, the previously empty table now has a candle in a tin holder sitting on it.

Continuity: The deputy takes one of three rifles from the gun rack, leaving two. In the next shot, however, there's only one gun left in the rack. A few shots later, there are two again.

Continuity: Tony jumps onto a brown horse and gallops into town to warn the sheriff. He doesn't stop on the way, but somehow arrives in town riding a black horse instead.

Continuity: When the aliens use their paralyzer on Doug and Tony, the time travelers are both bending forward. But in the next shot, they've somehow straightened up, despite being "paralyzed."

Continuity: A large blue vase in Galba's study keeps moving itself into different positions on the table. Several shots later, it's gone altogether.

Factual error: Doug and Tony time travel to one million years B.C., where they encounter dinosaurs. Not likely. All the dinosaurs were rendered extinct several million years earlier by an asteroid strike. There weren't any left by 1-million BC.

Continuity: Arthur ties a rope around the battlements and descends the tower's west wall. Tony then waves a torch to signal the soldiers to attack. He's standing at the same spot Arthur left from (the rope is still tied there), but in the long shot, his torch is waving from the tower's south wall instead of the west one.

Deliberate "mistake": Here, as in many other episodes, Doug and Tony are time-shifted wearing period (in this case, Viking) clothing. But they tumble into their next adventure magically wearing their left-behind 20th Century clothes again.

Factual error: Wogan the Viking probably shouldn't laugh quite so enthusiastically. When he does, several of his very 20th Century gold fillings are visible.

Revealing: This episode features borrowed scenes from the historical epic "Khartoum." Unfortunately, the movie footage is in uncorrected Cinemascope, making both the Arab and British armies look bizarrely skinny.

Continuity: The alien freezes Doug and Tony and escapes to the cellar. But when the shot cuts back to the time travelers, they're standing much farther apart and are posed in different positions than they were when first frozen.

Revealing: One of the supposedly frozen people in the general store is having a hard time standing still. He keeps swaying from one foot to the other.