California, Here We Come! - S4-E13
Visible crew/equipment: When "Mickey" takes his mother-in-law's suitcases to the back, just as Lucy and her mother sit down on the couch, we can see the bright shadow of something moving across the piano behind them. (00:06:50)
Visible crew/equipment: After Lucy phones Ricky and fibs about a man being on the fire escape, when Ethel and Fred rush in looking for Lucy, two actors' tape marks are visible on the floor where Fred and Ethel stand, beside the couch. (00:06:20)
Visible crew/equipment: When Lucy's sitting on the piano looking out the window at their new car, just as Ricky walks into the living room we can see the shadow of the mic equipment moving overhead, likely adjusting to the actor's position. (00:07:10)
Visible crew/equipment: When Ethel is about to open the door because Fred is too "busy" to do it, the shadow of moving boom equipment is visible on the wall to the right. Then after Ethel opens the door to find Lucy dressed in her starlet outfit, when Lucy walks in disappointed that she was recognized, in the next shot facing Ethel the shadow of the moving boom mic can be seen on the door. (00:10:40)
Revealing mistake: When everyone is examining the car that Fred bought, during a shot showing the front of the car, everyone's shadows are falling on the "street" behind the car, making it obvious that it's a painted backdrop.
First Stop - S4-E14
Revealing mistake: When the bed Lucy and Ricky are in moves as a train goes by look real close and you can see the wheels on it to help move it across the set piece.
Suggested correction: Not necessarily an error. My bed frame is on wheels.
Don Juan and the Starlets - S4-E18
Visible crew/equipment: After Fred fibs to Lucy about Ricky spending the night, when Fred walks over to the door just as Ethel walks in, we can see the shadow of the boom mic moving at the top, right corner of the screen. (00:18:30)
Audio problem: About 6 minutes in, a friend of Fred comes to the door and Ricky answers it. The man says Mr. Cardo instead of Mr. Ricardo, but Ricky says yes anyway.
Visible crew/equipment: At the very start of the opening shot, when Lucy is at the stove, there's a visible crew member at the right side of the screen.
Character mistake: When Lucy and Harpo are doing the mirror routine, Harpo grabs the horn on his belt with his right hand. For Lucy to create a "reflection", she would need to use her left hand but she uses her right hand.
Other mistake: Lucy walks out on the balcony and sees Van Johnson sleeping on a lounge chair by the swimming pool. The photographic backdrop shows automobiles, trees, mountains, the Roosevelt hotel and many other buildings of Hollywood but when Lucy goes to the pool and sits on the lounge chair next to Van Johnson, the background is just a plain painted backdrop with no cars, trees, mountains or buildings. This plain backdrop also appears in the episode with Harpo Marx.
First Stop - S4-E14
Visible crew/equipment: The gang stop at the roadside diner was obviously shot in-studio as the bottom seam is quite visible behind the car, and the backdrop shakes a little when Lucy opens the car door.
First Stop - S4-E14
Other mistake: When the Ricardos and Mertzes arrive the first time at One Oak Cabins and Cafe the full moon is amid the tops of the trees. When they arrive the second time the full moon is in exactly the same location amid the tree tops as the first time they arrived. Fortunately, hours later when they make their sneaky escape from the cabin and return to their car the moon is no longer in view.
Deliberate mistake: After Fred and Ethel arrive at Richard Widmark's house dressed as mental hospital staff, when Lucy hugs Ricky and holds out the grapefruit for Widmark to sign, the last closeup of the adorable St. Bernard is a flipped shot - note the chair in the hallway which should be to the left of the table, but is now to the right of the table. (00:25:05)
Continuity mistake: When Ethel asks her old friend Billy to take a group photo of her with Fred and the Ricardos, Ethel poses with everyone as Billy takes only one shot of them together, but when Lucy reads the newspaper article Ethel is posed differently in the cropped "group" picture. (00:12:15 - 00:16:35)
Plot hole: How does Lucy get back up to the apartment bound and gagged? It would seem impossible to hop up all those steps, possibly take several hours if not all night.
First Stop - S4-E14
Deliberate mistake: When they are counting the yards to Aunt Sally's pecan pralines, they count down 300 yards, 200 yards, 100 yards so fast they would have to have been speeding pretty fast to go hundreds of yards within seconds.