In "Lucy Fakes Illness," Ricky claims that he and Lucy have been living in the Mertzes building for 5 years. In later episodes it was 9 years, then the exact date, August 6, 1948. By 1954 it had suddenly leaped to 12 years.
In "The Ballet," Fred mentions his ex-vaudeville partner was Ted Kurtz but in "Mertz and Kurtz," his ex-vaudeville partner changed to Barney Kurtz.
In "The Marriage License," Elizabeth Patterson whom played Mrs. Willoughby (Mother) was later hired to play the recurring role of Mrs. Trumbell.
In "The Publicity Agent," when Ricky enters the Waldorf suite as "Tiger," he is wearing special platform shoes that elevated him more than 6 inches.
In "The Girls Want To Go To A Nightclub," the Mertzes celebrate their 18th wedding anniversary but in "The Courtroom," the Mertzes celebrated their 25th wedding anniversary. In "Ricky Minds The Baby," the Mertzes mention they are married 23 years. In an earlier episode, "Vacation From Marriage," Ethel says "he hasn’t varied 5 minutes in 22 years." Then in the following scene, Fred says that "after 25 years of marriage…."
In "Ricky Loses His Voice," when Ricky tells pianist Marco Rizo to "call Lucy and tell her I’ll be home right away" he slips and says "Okay, Des" in stead of "Okay, Ricky."
In "The Club Election," Caroline Appleby’s name was actually Lillian Appleby first but then was changed to Caroline in later episodes.
In "No Children Allowed," Ethel makes reference to that "old Mrs. Trimble who lives upstairs," not Mrs. Trumbull.
In "Sales Resistance," Fred buys a new Handy Dandy Washing Machine for Ethel but thirteen episodes later in "Never Do Business With Friends," the Ricardos sell their old washing machine to the Mertzes.
In "Lucy Tells The Truth," the Ricardos apartment number was changed from 3-B to 3-D to fulfill a joke about "3-D".
In "Redecorating The Mertzes’ Apartment," Lucy says to Ricky "Let’s paint the furniture and reupholster the old furniture," instead of "Let’s paint the apartment and reupholster the old furniture."
In "The Camping Trip," Ethel knew how to drive but in "Lucy Learns To Drive," Ethel asks Lucy to teach her how to drive.
In "The Passports," according to the Postal Service, there is no West Jamestown, New York where Lucy learns she was born.
In "Paris At Last," the Desilu prop department made an error by placing a bottle of California wine on the table in the sidewalk café scene.
In "Lucy And Superman," technically Little Ricky was only four not five, since he was born on January 19, 1953.
In "Little Ricky Gets A Dog," the no-pets-allowed clause in the Mertz apartment house apparently didn’t affect the Mertzes in "The Diet" where we see their dog, Butch or Mrs. Trumbull whose cat was mentioned on at least one occasion.
In "Lucy Does The Tango," the tango/eggs routine resulted in the longest laugh ever recorded on "I Love Lucy"—65 seconds.
In "The Saxophone," Lucy claims to only know how to play "Glow Worm" but in "Ragtime Band," she claims to only know "Sweet Sue".
In "Lucy Thinks Ricky Is Trying To Murder Her," when Lucy and Ricky are dancing you can see part of the bedroom at the left side of the screen.
In "The Seance," you can see the shadows of the production staff on the walls in the background.
In "Breaking The Lease," Ethel plays such songs as Sweet Sue on the piano but in "Lucy’s Club Dance," Ethel definitely can’t play the piano. In "Ragtime Band," Ethel can’t play the piano there either but in "Lucy Meets The Mustache," Ethel plays the piano accompaniment for Edie’s song.
In "The Gossip," Ricky mentions that Bill and Grace Foster lived in 3-B but later in "The Anniversary Present," they lived in 2-A.
In "Lucy Does A TV Commercial," when Ricky is singing, you can see Lucy standing in the apartment setup, watching.
In "Lucy’s Show Biz Swan Song," you can see Lucy untying her dress to make her pants fall down.
In "Lucy And Ethel Buy The Same Dress," Ethel’s middle name is Louise but in "The Million-Dollar Idea," Ethel’s middle name changed to Roberta, then in "Ethel’s Home Town," it became Mae.
In "The Tour," Lucy and Ethel get off the bus to get a grape fruit off of Richard Widmark’s tree but when the bus pulled away it shows them still on the bus.
In "The Fox Hunt," as soon at Lucy gets on the horse, the horn blows and the horse takes off. As it takes off, in between some bushes, you can see someone pulling the horse.
In "Lucy And Bob Hope," in the routine for "Nobody Loves The Ump," Lucy gets her cleats stuck in the new floor section so she has squares on the bottom of her shoes. Right after the routine ends, the tile squares suddenly disappear from her feet.
The Ricardos phone number seemed to change a lot. First it was, Murray Hill 5-9975, then it became Circle 7-2099, and then again to Murray Hill 5-9099. The reason for this was to ensure that fans would not call the number and actually get a hold of someone.
In "The Ricardos Visit Cuba," Uncle Alberto ruins his hat while listening to Ricky and Little Ricky play the drums. Seconds later his hat is as good as new.
In "Lucy Wants to Move To The Country" 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. When the Ricardos 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.
In "Lucy Thinks Ricky is Trying To Murder Her," you can see Lucy purposely drop the frying pan she was using to protect her from Ricky.
In "The Marriage License," Lucy and Ricky drive a car to Connecticut to "renew" their vows but when the Ricardos and Mertzes gets ready to go to Hollywood, they state that they'd never had a car before.
In "Too Many Crooks," when the girls tell Fred which way the REAL Madam X went, Fred runs the opposite way, which would take him right out Lucy's living room window!
In "Redecorating The Mertzes Apartment," after the feather fiasco, Lucy and Ethel are in the Ricardo's living room, you can see a feather float down from above the set!
In the "I Love Lucy Christmas Show," when the gang is reminiscing about old times, Fred turns on the lights on the tree and within 10 seconds, they pop back off. Pay attention to Lucy during this scene, she deliberately pulls the plug on the tree, causing the lights to pop off. She covers up by laughing.
In "Too Many Crooks," Lucy has a little trouble with her dialog, saying, "Ricky, have you seen your silvo-silver cigarette case?"
In "The Ballet," when Lucy's girdle falls off, you can see her untie it from behind!
In "The Dancing Star," Lucy is in the dressing room getting ready when she accidentally calls Ethel "Viv."
In "Ricky Has Labor Pains," Lucy is reading Life Magazine where on the cover "I Love Lucy" is mentioned.
In "Ricky Asks For A Raise," Lucy, Fred, and Ethel make seventy-five bogus reservations under various names and disguises for Xavier Valdez’s opening and when they learn Ricky Ricardo isn’t playing there anymore they leave. When Lucy, Fred, and Ethel show up for a reservation of 12 they’re seated at a table set up for 4.
In "Lucy Goes To Alaska," one of the times Lucy falls off the hammock, she throws the blankets and pillows back on but in the next scene the whole hammock is made up beautifully with everything arranged properly.
In "The Publicity Agent," when Ricky enters the Waldorf suite as "Tiger," he’s wearing special platform shoes that elevated him more than six inches.
In "First Stop," before Lucy gets out of the car to read the sign about the pecan pralines, her coat his white. When she’s running to the sign it’s black and when she gets back into the car is white again.
The "Lucy Is Jealous Of Girl Singer" episode appears to have been mistitled. Rosemary was clearly a "girl dancer," not a "girl singer."
In "Ethel’s Home Town," Irving Bacon who played Ethel’s father was only eight years older than Vivian Vance and seventeen years younger than William Frawley.
In "Sales Resistance," Lucy says that the handy-dandy kitchen helper costs $7.98, but later in the episode, she says that it costs $7.95.
In "The Gossip," Robert Jellison appears as the Milkman. Later when the gang goes to Hollywood he returns as the bellboy, Bobby....Does he have a twin??
Richard J. Reeves plays Hank in "Be a Pal," then he appears as a neighbor, then an electrician.
In "Lucy Does A TV Commercial," Jerry, Ricky's agent, shows up as Joe in this episode.
In "Ricky Needs An Agent," everyone claims that Ricky doesn't have an agent. What happened to Jerry??
In "Lucy Hunts Uranium," Lucy, Ricky, Fred, Ethel, and Fred McMurray are racing each other to get to the claim office to turn in uranium. Fred and Ethel are driving a Cadillac Convertable from 1925....didn't they get rid of that in "Lucy Learns To Drive"??
In "Little Ricky Gets Stage Fright," Little Ricky is scared to play his drums in front of an audience but two episodes before Little Ricky doesn't even know how to play.
