A character is counting something but loses his place and has to start over again. Alternately, another character may distract the first one (either accidentally or on purpose) by mentioning different numbers, confusing the counting character into picking up the count from those instead. The latter is the heart of the Quick Change con.
In Gaston Lagaffe, the eponymous Gaston drunkenly tries to count bottles arranged in a circle around him and turns around for a long time, counting the same bottles over and over again.
A very dark version in the Preacher comics, where the eponymous Preacher uses his command voice to force someone to count millions of grains of sand.
In Abbott and Costello in the Foreign Legion, Lou is counting out potatoes while Bud is complaining about their recent hardships. Every single time Bud mentions a number in his tirade, Lou resumes his counting from that number.
In the Bud Spencer & Terence Hill movie Go for It (1983), Rosco and Doug sabotage supervillain K1's attempt to detonate the "K-Bomb" which is supposed to eradicate all numbers on the world by distracting K1's spoken countdown before the rocket carrying the bomb is taking off, causing it to miss it's target and exploding harmlessly in space.
In Flatterland, Victoria Line (yes, an actual line) pranks the Space Hopper by pretending that she's been counting all of the thousands of 4-d "kissing spheres" combinations when he wakes up, then "losing count" and starting over from 1. Victoria loses count at 196,554 out of the 196,560 "kissing spheres" (meaning spheres of the same size that are touching). The maximum number of spheres of the same size that can touch one sphere in 24-dimesional space is 196,560; so Victoria made it look like the Hopper interrupted her just six spheres away from counting all of the 24-dimensional spheres.
The Adventures of Brisco County, Jr.: Pete Hutter is strapped in to a Chinese Water Torture device and counts the drops hitting his head in an attempt to stay sane. He gets up to 1,640, but after a visit from his torturer he discovers he's lost count and starts over at 1.
The Benny Hill Show: Benny as an accountant is counting a big pile of one pound notes. He's almost to the last one, then gets distracted by a girl in a short skirt bending over a file cabinet. After she leaves he starts to continue, but realizes he doesn't remember where he left off. He's forced to start over from the beginning. (Mildly NSFW image)◊
In The Brady Bunch, Alice is counting a pile of jelly beans. She stops a moment to explain to the others who come by that she's trying to approximate the number of beans in a jar at a local store, to win a prize for guessing the correct number. She then resumes counting, but now can't remember if she was up to three-thousand two-hundred-and-ninety-six or two-thousand three-hundred-and-sixty-nine (or some such). While the others debate about the number, Alice calmly empties out the sample jar, and begins again with 1, 2,...
On Corner Gas, Emma is at the gas station counting jelly beans for a "Guess the amount in the jar" contest. At the same time, Brent rings up a customer and tells him "$8.50", which makes Emma think she's counted to 850. She gets confused and starts over. Later on she has to start over again when she discovers that Karen's been eating some of the jelly beans she counted. They finally decide to abandon the counting and just lie about the number.
Mighty Morphin' Power Rangers episode "A Pressing Engagement" has Jason trying to make a bench press record, but Ernie keeps losing count (for various reasons) and has to start all over again.
An episode of Wilmot has him trying to beat a record by running laps around his garden. A pair of girls make his friend (who is keeping count) lose count.
Wimzie's House. A subplot in one episode has Russo spending much of the entire day counting the pennies he has in a jar, but other activities in the house keep causing him to lose count; a couple of times, Lou Lou is able to help him pick up where he left off. In the end, he finally finishes counting all the pennies, only to spill the jar.
In an episode of a later version of Get Smart, Max is counting a large stack of money. The first time, he is thrown off by someone mentioning that if they don't get all the money back, they're going to Leavenworth - which Max hears as eleven. The second time, near the end of the episode, he's almost finished counting, when a night worker mentions the time being four in the morning, and Max starts counting from four.
When counting a 10 count due to someone being outside the ring, or a 5 count to break an illegal hold, if the referee is interrupted he starts the count over from one. You'd think it's because he's easily distracted, but no; it's in the actual rulebook that he has to start over.
In The Goon Show episode "The Canal" Bluebottle has to pay out a large sum of money, which he counts out in pennies. As the count nears the end there is a crash of piles of coins hitting the floor, and he starts again from one.
The bomb in Starship Titanic. When activated, it starts a countdown (out loud) and makes so much fun of the player's attempts to figure out its disarm code that it frequently loses count and has to start over again from the top. This also carries over into the novelization, where the bomb always gets confused and starts over when somebody interrupts its countdown.
In World of Warcraft, there's a gnome in the Shrine of Seven Stars counting the gold in the bank vault. He regularly gets confused and starts over.
In one Arthur, King of Time and Space strip, a character is counting a deck of cards, and keeps getting thrown off by numbers drifting over from a conversation happening in the background. In the end, he coincidentally ends up on the same number that he would have done if he hadn't been distracted.
An episode of Arthur has an episode where Brain attempts to prove centipedes do not have have 100 legs, despite their name. His method of proving this? By counting them one by one. We see him getting interrupted in one scene and having to start all over again.
In Chowder, Schnitzel goes to the bank with Chowder and the woman in front of him wants to make a deposit, in change. As the teller keeps counting, Chowder keeps interrupting and mentioning numbers, which causes the teller to lose count, and Schnitzel to lose his patience.
This used to happen in DuckTales usually with one of Scrooge's nephews interrupting him and making him lose his count.
The Fairly Oddparents: The time when Timmy wishes to try to have the step dad of another kid. Doing pushups... after Timmy interrupts he has to start doing the thousand pushups from zero.
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