Anything that can go wrong will go wrong.
Associated with Murphy, an engineer on Stapp's rocket-sled programme around 1949 to 1951, though the exact origin is disputed. The lesson usually drawn is constructive: design so that the wrong assembly is impossible, since if it can be done wrong someone will do it wrong.
In practice
- A drug cabinet holding two vials that look alike will produce a wrong dose, and the fix is different packaging rather than more care.
- A connector that can be plugged in two ways will be plugged in the wrong way, which is why the correct design is a connector that only fits one way.
- Possible explanations
- One explanation is selection in memory: failures are salient and near-misses are forgotten. Another is combinatorial, since a complex system has many more ways to fail than to work, so with enough trials the possible failure occurs.
- In its favour
- Read as a design rule it is sound: a component that can be assembled the wrong way eventually will be, which is why connectors are keyed and interlocks exist.
- Against it
- As a law it cannot be tested, since it says nothing about how often or over what period, and it can encourage fatalism where the useful reading is preventive.
- Standing
- Exact origin disputed; the design-for-error reading is an interpretation rather than a documented original statement.
- Further reading
- Wikipedia
- Subject
- Risk & Reliability
- Essay
- How much caution, and the jokes about its limits