In "Lucy Wants to Moves To The Country" #167, when Lucy, Fred, and Ethel dress up as gangsters to try to get the deposit back, there is a Grandfather clock by the front door and there are plates on the fireplace mantel. Then when the Ricardo's actually move in, the spot where the clock was is empty, but a few episodes later, the same Grandfather clock is back next to the front door, and the same plates are back up on the fireplace mantel. I guess the Ricardo's shopped at the same store as the previous owners.
In "The Ricardo's Visit Cuba" #162, Uncle Alberto ruins his hat while listening to Ricky and Little Ricky play the drums, and seconds later the hat is as good as new.
The Ricardo's phone number seemed to change a lot. First is was, Murray Hill 5-9975, then is became Circle 7-2099, then again is changed to Murray Hill 5-9099. For this blooper, though, there is an explanation. To ensure that fans would not call the number and actually get a hold of someone, the phone company would tell them which numbers they could use that would not be already assigned.
In episodes originally broadcast only a month apart in 1952, Ricky mentioned that he and Lucy had lived in the Mertzes' apartment building for five years, then nine years; and then a few segments later, he gave the exact move-in date, August 6, 1948! By 1954, it had suddenly leaped to twelve years," says Bart Andrews in "The 'I Love Lucy' Book."
Episode #171, "Lucy Raises Chickens"; when Lucy, Fred, and Ethel are in the kitchen taking a break from rounding up the chickens, Little Ricky comes in from outside, you can see a hand from outside the "house" pushing him through the door.
In "Lucy and Bob Hope" #154, Lucy, Ricky, and Bob do the song "Nobody Loves the Ump". In this routine Lucy gets her cleats stuck in the new floor section so she had 1'x1' squares on the bottom of her shoes. Right after the routine ends, the tile squares suddenly disappear from her feet.
In "The Fox Hunt" #143, as soon as Lucy gets on the horse, the horn is blown, and the horse takes off. As it takes off, in between some bushes, you can see someone pulling the horse.
In "Tennessee Ernie Visits" #94, when Ricky is explaining to Lucy how they are going to let Ernie find the ticket in the hallway, Ricky is imitation Ernie finding the ticket and bends down to pick it up. The camera goes down too, and if you look at the bottom of the screen, you can see a cord running across the floor.
In episode #127 "The Tour," Lucy and Ethel get off the bus to get a Richard Widmark grape fruit, when the bus pulls away it shows them still on the bus.