Continuity mistake: Monique's position on the floor reverses while she's unconscious. When she first collapses, her feet point to the corner of the room. When she comes to, her head is toward the corner. When she gets up, her feet are oriented toward the corner once again.
Factual error: The truck transporting Number 6 and Nadia drives on the left side of the road. Fine for England, where the scene was shot - but they're supposed to be in Poland, where everyone drives on the right.
The General - S1-E6
Continuity mistake: Number 6 is stabbed in the hand during his struggle with the projectionist. After he's treated for the injury, not only is he placed in an unnecessary sling (his arm wasn't broken), he's also suddenly bleeding from the forearm, several inches above the area where he was stabbed.
Free for All - S1-E4
Continuity mistake: Number 6's bed tidies itself in the scene where the spying lamp is lowered from the ceiling. First the bedcovers are rumpled and disarrayed. Then in the next shot, all is straight with the blanket neatly folded at the foot.
Continuity mistake: While the gunrunner is untying the raft, he's holding a half-eaten green apple in one hand that wasn't there in the previous shot. It disappears again as the raft floats away. Then it comes back - but it has now become a red apple instead of a green one.
The General - S1-E6
Continuity mistake: At the Village cafe, the menu on the table turns itself 90 degrees between the master shot and the close-up. Number 6's hand position and one of the chairs also both change positions instantly.
The General - S1-E6
Continuity mistake: In The General's office, the professor pulls the sheet of paper he's been typing on out of the typewriter and inserts it head-first into the computer. When we cut to a close-up of the paper going in, however, it is now being inserted bottom-first.
Visible crew/equipment: When Number 48 jumps from the council table, there's a clear shadow on the left of someone holding up a still camera.
Free for All - S1-E4
Revealing mistake: Though there aren't supposed to be any "normal" automobiles in the Village, there are several visible in this episode, parked in the background. Look closely behind the campaign placards as the golf cart/taxi moves, and behind Number 6 as he leaves the exchange. Several more can be seen behind 6 when the cameraman approaches him.
Continuity mistake: Between shots, the bodies of both Kathy and the Kid change positions on the floor. Kathy's has moved away from the stairs, and the Kid's reverses directions.
Continuity mistake: In full shots, the judge's cigar is in his mouth. Every time a close-up is intercut, the cigar is in his hand.
Continuity mistake: Number 6 points the mirror out to sea and tries to send a signal. But when we see him from the POV of the security camera, he's pointing the signal mirror toward the Village instead. When the shot cuts back to him on the beach, the buildings are again behind him and he's facing seaward again.
Continuity mistake: In the final scenes, as helicopters are taking off from the Village, the bell tower that's always been there is visible in some shots, but in others, it has disappeared.
The Girl Who Was Death - S1-E15
Continuity mistake: When the bulldozer is on fire, the raging flames have died down completely in one shot, but are instantly burning brightly again in the next one.
Continuity mistake: Twice in this episode (once in the saloon and once in the sheriff's office), Number 6's coat is unbuttoned when he's inside, but has instantly buttoned itself up when he runs outside.
Do Not Forsake Me Oh My Darling - S1-E13
Continuity mistake: In Number 2's office, the Colonel stands at the bottom of the ramp. In the next shot, however, he's suddenly beside the tea cart instead.
Continuity mistake: Nadia's number badge is upside down when she serves Number 6 breakfast. It's turned itself right-side up when she leaves, though.
Continuity mistake: In long shots of the truck transporting Number 6 and Nadia, the crate they're supposed to be hidden in disappears from the truck bed.
Continuity mistake: When the lamp begins spying on him, Number 6's bed sheets and blanket rearrange themselves from messy to neatly tucked in between shots.
Free for All - S1-E4
Revealing mistake: Rover flies down from the Green Dome, and briefly reveals his true identity as a weather balloon. His air nipple and guide wires are both showing.
Answer: It's even more obvious than you think, you know who number 1 is in the very first episode. When 2 replies to the question "who is #1?" Change the way he answers from you are number one (in the monotone or accented answer to, "You are, number 6. The comma gives you the answer. #6 is #1. It's the tone of the answer.