Thou shall now kill.
(Inscribed in a stone tablet)

In the original Hebrew, change לא ("thou shall not") to לו .לו by itself means "to him" or "for him" or something similar (hard to explain exactly). But together with other words it is sort-of possessive, sort-of permission. It would become something like "For him is to kill".

There are actually places in the Hebrew original (but not in the 10 Commandments) where לא is normally read/understood to mean לו or vice versa, giving the sentence the exact opposite meaning from the way it is written, but with the same pronunciation. See for example Leviticus Chapter 25, Verse 30 where the phrase translated as "then the house that is in the walled city" is actually written in Hebrew לא which would mean "then the house that is in an unwalled city" but is traditionally read in Hebrew as לו and therefore translated (as per traditional Jewish reading of the verse) "walled" instead of "unwalled".

Add a non-type-able digit into the password of the suitcase known as "the football" which is always in the possession of a secret service agent in proximity of the President. If you don't let anyone know what you've done, you could possibly save almost 7 billion lives.

And since chaos, bedlam and destruction are natural byproducts of human life, you would thereby be responsible for all the harm we do from now through the end of time.

If I can only change a single keystroke of a single piece of information, I would have to change the original source for the most widely used form of COBOL to render the programming environment inoperable.

COBOL is still one of the most widely used languages for financial software due to several factors. Preventing the programs from operating would throw a large percentage of the world's financial markets into disarray, doing catastrophic (though, sadly, not world ending) levels of harm.

In the (very popular) language C; the difference between equals and not-equal is one character; the "!" characterSo

if ( X == Y ) LaunchAllBombs();

could be changed to do something

if ( X != Y ) LaunchAllBombs();

And "LaunchAllBombs()" will be executed precisely when it should not be.

This is true in other languages as well. Likewise, it could exist in banking:

if ( X == Y ) SellAllStocksImmediately();

vs.

if ( X != Y ) SellAllStocksImmediately();

Causing a market crash and the loss of fortunes.

I'm not saying there are any such routines, but taking a catastrophic path does exist in both defense code and automated trading code, in satellite code, etc. A terrorist would likely look for exactly that.

I will change one codon in the DNA sequence that lets mammals produce haemoglobin. Everyone dies quite quickly, as they run out of red blood cells containing functioning haemoglobin.

Based on the precedent set by Mike Scott's answer as I understand it:

Change the value of G or any other universal constant.

For instance, $G = 6.674×10^-11 N·kg^–2·m^2$ becomes
$G = 6.674×10^-91 N·kg^–2·m^2$
and the entire universe falls apart.

No detection or interference? Ok.

Wait until a major bank is running a permanent, large sum transfer from the Euro to the US dollar, and change the byte storing the exchange rate so that 1 Euro equals 1 quadrillion dollars.

They now have more dollars than had existed a second ago. The dollar is worthless. The US economy collapses. The global economy collapses. World hunger spikes. Panicked refugees spread across the world, overwhelming governments. Total societal breakdown occurs. Terrorists get hold of nuclear weapons in destabilized governments, bomb the world in the name of some religion. The survivors are wiped out by starvation, disease and fallout. Everyone Dies.

Simply change the defense condition alert status (DEFCON) of the United States armed forces from whatever level it is currently at, likely a 4 or 5, all the way up to DEFCON 1 and wait for someone to blink and start all out thermonuclear destruction.

DEFCON 1 condition means that nuclear war is imminent, it would not itself trigger the launch of any missiles, but it would trigger a lot of armed forces actions that could easily incite further action on the part of the other world powers.

For comparison DEFCON 2 has only been declared twice in history, for the Cuban Missile Crisis and briefly during the start of Desert Storm in the early 1990's.

Two thoughts:

1. 
   Add apostrophe’s everywhere that is grammatically incorrect.

These change’s aren’t going to cause major monetary loss, but any grammar nerd’s out there may well cause casualtie’s - and certainly some section of society will be in utter chaos. (Sorry - couldn’t resist...)

True, it’s not going to cause major societal upheaval (probably), but if shop signs, printed books, tattoos, religious texts, laws, and so on are all affected, that’s going to be a major annoyance for people.

2. 
   Change people’s legal gender.

New non-binary gender laws are now being introduced, but most countries still have the M/F option on things like passports, identity cards, birth certificates, and so on. Change all M to F, or vice versa, and make things a lot harder for half of the world’s population to verify their identities (assuming this change can be made in all languages).

Either that, or change all “Mr” titles out there to “Mrs”.

I would change nothing.

Single byte errors happen in computers every millisecond. Single character errors happened in accounting every minute long before computers. Single pictogram errors happened in ancient messages carved in stone.

As a result, no one trusts a single source. Computers check against each other's records. Finances use double accounting, so no single change can do as little as put money into an account. And everyone already expects written, printed or carved sources to contain errors.

Since I can't do anything, I might as well do nothing.

Add a digit to a critical economic index.

Dollar exchange value, an oil benchmark, US Federal debt, etc. An immediate 10-fold change wouldn't go unnoticed.

I would break the C++ language

C++ is widely used for development on specialized hardware. This hardware includes but is not limited to: Airplanes, Factory Automation, Databases, and military equipment.

Breaking C++ would cause most modern passenger aircraft to fall out of the sky. 9/11 used 2 aircrafts, even if nothing hits a major structure the damage would be catastrophic world wide.

Automated Factories would stop production and would cause global shortages in almost every product and could cause equipment failure in chemical plants which could harm people and destroy property.

A number of databases would stop working causing online services to collapse.

Advanced Military hardware would fail in the field, giving most insurgent and resistance movements an easy string of victories.

I would change the configuration files of the traffic light systems. 1 character change would be on the line that controls the green go light and everywhere would be set to Y instead of N.

It wouldn't cause much havoc because the power of 1 character is limited it would most likely just break the 1 system the 1 character is added to.

But if it was 1 line of code....that's explosive.

With 1 line of code I'd put an SQL injection line into the presidents login credentials so next time he logs in it triggers the scripts to launch nukes unbeknownst to him.

1 character has potential to stop things from happening.

1 full line of code has potential to destroy the world.

Hope that helps.

Every number and letter consists out of bites. Bites consist out of a string of one's and zero's. Change every bit that means "1" into "0".

Planes crash as every Electronic becomes useless, cars lose their steering and brake assists, all economies that have ties to digital connections collapse dragging the world into disarray and every research project and all information we havent written down is erased. All food normally grown through electronic assists breaks down and famine sweeps the world as refrigeration and food production collapses. Nuclear powerplants have massive failures as the quadruple redundancies with pumps and whatnot all fail simultaneouslu and meltdowns cover the earth. A new era dawns for the remaining humans as all electronics are distrusted, assuming they survive the food shortages and radiation.

Delete the Left-Pad Node.js project

The one keystroke here would be the [Delete] key.

With left-pad removed from NPM, these applications and widely used bits of open-source infrastructure were unable to obtain the dependency, and thus fell over during development and deployment. Thousands, worldwide.

Essentially, a larger portion of the internet ceased to function for a couple days, and if it was an unrecoverable deletion (Node un-un-published the project, an entirely unprecedented move), then it would cause a huge society upheaval.

Presumably, there are other projects like Left-Pad that could also cause a similar issue, Left-Pad is just unique for being so small (and the reason that the Node.js environment is awful: no one should have ever relied on this package in the first place and instead written their own function for it).

Alternatives ("manipulate 1 byte of text" instead of "one keystroke"):

• Renaming the package


• Making the package non-compilable (i.e. introduce an error)


• Changing the file/folder name of the package

If you're in an editor that allows you to comment out a line with a single keystroke, or perhaps paste a pre-copied line of text with a single keystroke, then you could cause a lot of havoc by simply editing the method which determines if a user is authorized to access a system, as outlined by Tom Scott in this video on "The (Fictional) Day Google Forgot to Check Passwords".

I can't think of a way to condense this down to one keystroke if you're not in such an editor, though really, most programs you'd use for hacking would allow you to paste from a thing already on the clipboard, and most IDEs have a line-comment keystroke.

Vandalize an important object in an unstable area. Even a one character change has a lot of potential to cause offense. Since you cannot be detected, people who revere the object would need to speculated about who committed the vandalism. If those who revere the object conclude a particular group is to blame, certainly societal chaos could follow, and perhaps even casualties.

For instance, vandalizing a gang tag could incite gang violence and vandalizing a monument like the Kaaba could prompt a diplomatic sanctions and incite violence from groups already prone to violence.