In the first episode "The Reluctant Stowaway", when Dr. Smith is trapped aboard the Jupiter II, he is wearing a Class A U.S. Air Force jacket with campaign ribbons etc. which he then takes off after the launch. In this episode, when he dons a jacket again, it is the non-descript jacket devoid of US military insignia that he wore in the opening scene of "The Reluctant Stowaway" instead, and which he continues to wear for the next few episodes.
When the Jupiter 2 rises and then exits the alien ship, the personnel at the window change from the exterior shot to the interior shot (four spread-out figures in the exterior, five people clustered together in the interior).
Will Robinson's hair varies from back and forth between several scenes as he gazes through the porthole window at his father and mother struggling outside the spaceship. First it is dry and neatly combed without any of his forehead showing. Then it is stringy and wet looking with parts of his forehead visible as though soaked with sweat. Then it is once more is dry and neatly combed while covering his entire forehead, only to be stringy and wet in another scene.
When John lasers through the alien hull, there is no beam. When he turns and shoots the alien, there is a beam. When he turns back and continues cutting through the hull, the beam is once again missing. When he finishes lasering the hull and scoots inside the Jupiter 2, the target point of his laser-rifle continues to burn unabated.
When Maureen rescues John from floating off into space, Will, Penny and Judy watch through a window standing one behind the other, respectively in that order. When the scene shifts to the interior, the children's positions change, with Judy and Penny standing shoulder-to-shoulder and Will standing behind them. Later on, Will's position shifts again from front of the line in one shot to the back of the line in the subsequent shot.
Commander John Robinson writes in his log about the harrowing first "24 hours" in flights but dates it October 21st which is 5 days after their liftoff that took place on October 16th.
Comets are balls of ice rather than balls of fire and intense heat. They only glow by reflected light from the nearest star.
In the opening scenes when John Robinson is floating outside the ship, he drifts in random directions, going back and forth, without any way of changing his direction.
When Maureen passes out outside the Jupiter 2, she slides down the side of the hull -- pulled by what? There's no gravity in space.
When John and Maureen Robinson pass out outside the ship, they slump against the side of the ship, but they really would have just drifted unconscious off into space, since there was no gravity.
After Maureen's line "We do need a man here while they're gone," Penny smiles at Will, and the shot dissolves into the next scene. Before the dissolve completes, however, Penny can be seen turning her head to look at the camera.
When John pounds on the exterior of the Jupiter 2 at the 7:36 mark, the hatch door quivers, showing itself to be of flimsy construction, unworthy of space travel.
While on the lower deck, Will Robinson says to the Robot: "I know my dad told you to watch me, but can't you look away for a minute?" Professor Robinson could not have ordered the Robot to do anything, since the Robot was programmed to respond only to Dr Smith's voice.
When the aliens go on the attack, white tennis shoes of the rear alien operator can be seen.
John Robinson states that the percentage of oxygen in the atmosphere inside the alien ship is "low" at 5.4%. Shortly thereafter he pronounces that atmosphere breathable, and Robinson, Don and Dr. Smith venture outside the ship with no breathing apparatus. In reality, 5.4% oxygen is less oxygen than is present at the summit of Mount Everest. Anyone not acclimated to such an atmosphere would quickly pass out and eventually die.