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Misplaced a Decimal Point

If a work of fiction has a character who's good or supposedly good at mathematics, he or she will inevitably screw something up somewhere down the line. When this happens, the project the group is working on comes crashing down, sometimes literally, and the mathematician will make a gesture of embarrassment and say, "I must have misplaced a decimal point."
This doesn't happen nearly as often in real life among mathematicians and scientists, since decimal notation is usually not the easiest way to write something. More common are radicals, fractions proper and improper, constants (such as pi or the natural base), and scientific notation ^{note }Though technically, using the wrong exponent here is equivalent to misplacing the decimal point, sometimes over many orders of magnitude. It does, however, happen at times—not very often (as traditionally one keeps track of things in terms of integer multiples of the lowest common currency, such as integers of cents instead of decimals of dollars)—in finance.
The most likely cause of the prevalence of this near iconic catchphrase for a mathematics error is a combination of Writers Cannot Do Math and Everybody Hates Mathematics. Misplacing decimal points is a characteristic of elementary school and middle school arithmetic. Thus, this will most commonly show up in works aimed at children, for the reasons that it doesn't take much for a kid to consider a term as advanced mathematics and that the writers themselves, being kids at heart, are the writers least likely to have taken advanced mathematics.
Subtrope of E = MC Hammer. Compare with Carry the One, the other arithmetic mistake common in fiction. See also Mismeasurement.
Examples:
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Comic Books
 In Fantastic Four comics, Victor Von Doom misplaced a decimal when planning a clandestine scientific experiment. When his roomate Reed Richards tried to point it out to him, Doom went into a rage at the suggestion that he could have made a mistake and went ahead anyway. The experiment literally blew up in his face, scarring and forever embittering him against Richards, convinced that he must have changed the calculations in an attempt to sabotage him.
 In the Lucky Luke episode Outlaw, the Daltons (the original ones, not the nephews from the later stories) try to divide their loot among themselves. Having had no actual schooling, they fail horribly, turning a simple long division into a mathematical nightmare. Bob Dalton uses his gun to place a decimal point "to simplify things".
 At the end of Superman: Red Son Luthor's plan to destroy Superman and Take Over the World seems to have succeeded without a hitch...only for it to be revealed that Superman is still alive due to Luthor having misplaced a decimal point when he calculated Supes' density. However, due to Luthor having made Superman see the error of his ways as well as the world being a better place under Luthor's rule, he decides to let Luthor think he won and live the rest of his days in anonymity.
 Superman, as seen in the page image for Writers Cannot Do Math, thinks 20 times 16 times 10 equals 32,000 (the correct answer is 3,200).
Film
 Invoked in all versions of The Producers. When begging Leo to not report his small scale embezzlement at the beginning, Max tells him he should just misplace a few decimals.
 The given cause of the plan's failure in Office Space was that Michael misplaced a decimal point, which results in far too much money being siphoned off. He claims that he always makes similar minor mistakes.
 More justified than most examples due to the simple fact that Michael wasn't doing math, he was writing a computer program that involved math. It's much easier to make a mistake like this under such conditions.
 Muppet movies:
 In The Muppet Movie, Kermit tries to buy a car at a rather disreputable dealership, but can't afford any of them, until Sweetums swats a fly which creates a decimal point in a price tag; the dealer finds himself selling a $1195 car for $11.95, after paying a $12 tradein on the Muppets' old lemon.
 At the end of the telethon in The Muppets, the titular characters have raised $9,999,999  one dollar short of the $10 million they need to save their theater. As Richman gloats, Fozzie bangs his head against the money counter in despair. The display flickers, rolls, and resets... revealing that the decimal point wasn't displaying correctly, and that they actually raised only $99,999.99.
 Kellys Heroes uses this as a throwaway gag when Crapgame calculates the value of the Nazi Gold that Kelly is trying to steal. He initially comes up with $1.6 million, but two scenes later announces that he misplaced a decimal and it's actually $16 million.
 In Kill Me Again, the broke private eye played by Val Kilmer desperately needs some dough to get out of town. He checks his bank balance by phone and is agreeably surprised to find he has $732, then crestfallen to learn that the clerk meant $7.32.
Literature
 In one of the Harold Shea stories by L. Sprague de Camp and Fletcher Pratt, where magic has a mathematical basis, one of the characters has to prove his magical skills by invoking a dragon. The guy accidentally shifts the decimal point two places to the right and summons 100 dragons. Luckily they're vegetarian.
 A later attempt to perfect the spell produces a dragon approximately 0.01 times the size of a normal dragon. It's ten inches long, breathes fire, has a sting in its tail...and was summoned into a cage made for the normalsized dragon the characters were hoping to get. It promptly escapes by flying between the bars...and it's not happy.
 It is, however, adorable.
 In Unseen Academicals , the Bursar of Unseen University is said to regard the decimal point as a nuisance. Inevitably, this leads to Ponder Stibbons taking over his responsibilities. Although being utterly mad may have more to do with it (not that that stopped him before) as he had previously been shown to be very mathematically adept.
 As of The Science of Discworld, the Bursar is now too mathematically adept to bother with decimal points, or real numbers. "Of course, he was a natural mathematician, and one thing a natural mathematician wants is to get away from actual damn sums as quickly as possible and slide into those bright sunny uplands where everything is explained by letters in a foreign alphabet".
 In The Purchase of the North Pole by Jules Verne, the antagonists' plan is doomed to fail from the very beginning, because, as it is revealed in the end, the mathematician responsible for it was interrupted while writing down Earth's perimeter, which caused him to effectively move the decimal point three places to the left.
 In "Homo Sol", a short story by Isaac Asimov, an alien scientist mentions an occasion when one of his students thought he'd disproved a longstanding theorem. It turned out he'd misplaced a decimal point in an exponent, meaning his result was 10000000000 times too large.
 In the Star Wars Expanded Universe XWing: Mercy Kill, main character Voort saBinring is a genius mathematician using his skill for strategy and tactics both in starship combat and in commando situations. He develops one of these vaudevillian schemes to capture the villain (of course, the reader is not told the details because otherwise the scheme would fail) but when executing it, he decides he had "missed a variable" when they kidnapped a person who was to be kidnapped by other people to attract attention.
Live Action TV
 In Mystery Science Theater 3000: The Undead, Tom Servo misses a single question on the tests given to the cast by the Brain Guys (which everyone else fails horrendously but Gypsy, and it's suggested via Overly Narrow Superlative that she'd have failed if they'd been able to decide on a control set) for this reason.
 Played with in Red Dwarf: White Hole, Holy's IQ has been significantly increased (to 12000) in exchange for exponentially reducing her lifespan. When looking at her new lifespan, the screen displays 345 before she realizes "The decimal point, where's the decimal point?" She then discovers that she has 3.41 minutes left to live.
Video Games
Webcomics
Western Animation
 In Ed, Edd n Eddy, Edd seems to have made this error on a few occasions.
Real Life
 There's an urban legend that spinach is said to have a high iron content because once upon a time, someone misplaced the decimal point, and the figures on modern nutrition labels are because every single person in every nutritional agency in the world since is just making computations based on that figure, not doing tests of any kind. This is, of course, false.
 Supposedly, the myth of Atlantis got started because Plato did the equivalent of misplacing a decimal point in his memoirs — if he had recorded the eastwest distance as exactly onetenth of what he described, it would point directly to Crete. Decimals hadn't been invented then, but there was confusion about the meaning of a particular hieroglyphic, mistaking 100 for 1000.
 A Calorie is 1000 times as much energy as a calorie. That is, the "Calorie" seen in American nutrition charts is actually a kilocalorie. This has to rank among the stupidest units of measurement (i.e naming conventions) that exist. It seems deliberately designed to confuse and lead to errors.
 Isaac Asimov also mocked this concept in one of his scientific essays, and proceeded to use kilocalories (abbreviated kcal) for the remainder of his calculations.
 Which has led to the fallacy: "You need one calorie to heat up 1 g water (or another drink) one degree celsius. One liter of a softdrink equals roughly 1000 g, but contains less than 500 calories. So if you drink a cold softdrink, your body will burn more calories than the drink contains to heat it up to body temperature!" Unfortunately, the drink really contains less than 500,000 calories.
 Something similar can happen whenever someone mentions a "billion" of something. In the United States it means a thousand millions, in other countries it means a million millions (a thousand milliards).
 It seems the people at Verizon have this problem. Misplacing the decimal can be a real problem when you're talking about money.
 This was probably much more common in the past, as slide rules don't give you a decimal point except in a very limited range of calculations. You're expected to place it yourself from context. (It's how a simple sliding scale manages to be so versatile — you make exactly the same motions to multiply something by 42, or 4.2, or 42,000.)
 This can happen nowadays as well if you use the scientific notation and calculate the exponent separately. for example 7*10^5 times 3*10^7 can be simplified to 7*3*10^(5+7) or 21*10^12 adjusting the mantissa yields 2.1*10^13 forgetting to increment the exponent while adjusting will lead to the trope.
 In a brilliant aversion of Stupid Jetpack Hitler German physicist Walther Bothe, who worked at their atomic bomb project, did exactly this when he determined the neutron freeflight length in graphite to be almost ten times smaller than it was in reality, forcing German nuclear project to use much more expensive and finicky heavy water instead of cheap and easily available graphite.
 A pilot on a flight from Maraba to Belem, on reading his flight plan, found his course was written as "0270". He interpreted it as 270 degrees (west), but in the flight plan it was assumed there was a decimal before the last digit, so the course was actually 027.0 degrees (roughly northnortheast). The plane ended up crashing in the jungle hundreds of miles from Belem when the fuel finally ran out; 13 people died.

