Rendezvous with Yesterday - S1-E1
Character mistake: When Tony talks to captain of Titanic he states he was born in 1938, but in episode 4 "The Day the Sky Fell In" it is December 1941, he should be 3 years old, but as a young child he is about 12 years old.
Continuity mistake: 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. (00:41:15)
Factual error: When Ulysses opens his mouth while speaking, modern dental fillings are in his teeth.
Continuity mistake: After prying off the lock to the storeroom with the pry bar, Doug drops it on the ground, but the next shot of him entering the storeroom shows it still in his hand.
Continuity mistake: A large blue vase in Galba's study keeps moving itself into different positions on the table. Several shots later, it's gone altogether. (00:20:00)
Visitors From Beyond the Stars - S1-E18
Continuity mistake: An escaping Tony leaves the cell door standing open. But when the shot reverses angles, the door has mysteriously closed itself. (00:22:20)
Visitors From Beyond the Stars - S1-E18
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. (00:05:20)
Visitors From Beyond the Stars - S1-E18
Continuity mistake: 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. (00:28:30)
Continuity mistake: 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. (00:10:00)
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. (00:45:30)
Suggested correction: A comment, not a correction to your astute and correct observation about the Time Tunnel. Aside from it being episodic television that allows characters to look the same in all situations, I believe Tony and Doug keep their clothing, hair length, lack of facial hair, etc, because the Time Tunnel "remembered" the exact disposition of Tony and Doug when they first entered the Tunnel, allowing them to keep their clothing, hair length, etc., as they slip through time. Any "Time Tunnel" aficionados wishing to add more to this would make a fascinating discussion.
A valid speculation with which I agree, but as the poster himself points out, not a correction.
Visitors From Beyond the Stars - S1-E18
Continuity mistake: 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." (00:01:45)
Continuity mistake: 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. (00:05:05)
Revealing mistake: 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. (00:19:30)
Visitors From Beyond the Stars - S1-E18
Continuity mistake: 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. (00:31:20)
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. (00:09:30)
Raiders From Outer Space - S1-E29
Revealing mistake: 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. (00:14:15)
Visitors From Beyond the Stars - S1-E18
Continuity mistake: 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. (00:12:45)
Revealing mistake: 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. (00:44:50)
Visible crew/equipment: At the very beginning of the episode, Nero's ghost appears and raises a knife to attack the heroes, the wires used to move the knife across the room are visible most of the time.
Factual error: The Vikings in this episode are fighting while wearing horned helmets. Contrary to popular belief, Vikings never wore horned helmets in battle. Some theories state the horned helmets were worn during special ceremonies, but never in battle or daily life.