Informal adage; risk-management heuristic

Murphy's Law

Expect failure paths before they happen, then design checks and backups so small mistakes do not become big disasters.

Popularity

Usefulness

Aliases

Fourth Law of Thermodynamics (humorous / informal) / no widely accepted formal scientific alias.

Domains

Engineering, safety design, reliability engineering, project management, operations, everyday decision-making

Definition

Murphy's Law is the informal saying: “Anything that can go wrong, will go wrong.”

Systems should be designed with the assumption that errors, failures, misuse, bad timing, and human mistakes are possible.
Its practical value is not pessimism; it is preparation.

When a process has a weak point, unclear instruction, fragile dependency, or possible human error, that weakness may eventually cause failure.
The rule encourages people to identify failure paths early and design safeguards, backups, checks, and fail-safe mechanisms.

Before deploying software, a team assumes that configuration values may be missing, network calls may fail, and users may enter unexpected input. They add validation, retries, logging, rollback, and monitoring.

Example: The commonly repeated origin story links Murphy's Law to U.S. Air Force rocket-sled tests in 1949, involving Capt. Edward A. Murphy Jr., John Paul Stapp's test team, and incorrectly wired sensors or transducers.
Why it fits this rule: A technical setup had a possible incorrect wiring path, and that path reportedly happened, causing failed data collection.
Verification status: Partly verified but historically disputed. U.S. Air Force-related sources repeat the Stapp/Murphy rocket-sled account, but word-origin research notes that the exact wording, first use, and press-conference story are not fully documented. (Arnold Air Force Base)

Do not treat it as a scientific law or a guaranteed prediction.
Do not use it as an excuse for poor planning.
Do not use it to claim that failure is unavoidable.
Do not confuse it with bad luck; the useful version is about preventable risk.
Do not overapply it so heavily that it causes unnecessary fear, delay, or over-engineering.

Invented by: Disputed. Commonly associated with Capt. Edward A. Murphy Jr.; John Paul Stapp helped popularize the phrase in the aerospace testing context. Earlier similar sayings existed before Murphy.
Year of invention: Commonly associated with 1949, but the exact origin is disputed. Word-origin research identifies a certain printed use in 1951 and broader aviation-safety use by 1956. (Wordorigins.org)
Country / context of origin: United States; aerospace testing, military aviation safety, and rocket-sled experiments.

Murphy's Law is not a formal scientific law.
Its practical basis overlaps with reliability engineering, safety engineering, human factors, and defensive design.
The Air Force account presents it as a risk-mitigation mindset: think through what could go wrong before a test and act to counter those risks. (Arnold Air Force Base)
Historical evidence supports that similar “what can go wrong will go wrong” ideas existed long before the modern Murphy name. (Wordorigins.org)

Expect failure paths before they happen, then design checks and backups so small mistakes do not become big disasters.