Good analogy needed: Sec issues due to different coders implementing the same features in different ways for the same app

Here's an idea for an analogy that I think is fairly accurate while generally understandable:

A bank requires two forms of ID to get a loan: a driver's license and a birth certificate. Bank employees Alice and Bob are lazy in different ways: Alice always stamps "driver's license verified" without checking, while Bob always stamps "birth certificate verified" without checking.

Individually this is bad but not too bad -- anyone applying with forged documents would get caught by the one check the employee still does perform. But one day Alice is running late, stamps "driver's license verified" on a form, and leaves it for Bob to finish up. Bob sees the form, assumes Alice actually verified the license, and stamps "birth certificate verified" without checking like he always does. The loan is approved, without either form of ID having been checked.

The situation is: people working independently without coordination, to design functionality meant to be useful locally, but when combined, created a disaster.

The first historical references that come to mind:

• the chaos of the UK rail system where each train line owner ran their own tracks, track sizes, trains, and timetables (and sometimes, their own clocks)
• the first New York fire-fighting companies where they fought for customers (sometimes literally, with fire and fisticuffs)

Both make for fun stories and exciting visuals.

The solution to both was central oversight and central planning, which is the same solution to Open Source fiascos, like the one Moodle experienced.

Here's a perfect example: the loss of the Mars Climate Orbiter. https://www.wired.com/2010/11/1110mars-climate-observer-report/

To quote:

A NASA review board found that the problem was in the software controlling the orbiter's thrusters. The software calculated the force the thrusters needed to exert in pounds of force. A separate piece of software took in the data assuming it was in the metric unit: newtons.

This resulted in a disagreement of about 4.5 times the right amount. The satellite entered the atmosphere much too deeply and burned up.