Types of human error: slips, lapses, mistakes and violations
Not all errors are the same and they are not fixed the same way. James Reason's classic classification, explained in plain terms.
Also called: Reason taxonomy · error classification
Psychologist James Reason proposed the classification now used across aviation, healthcare and industrial safety. His starting idea is simple and powerful: what separates one error from another is not how serious it was, but where the chain broke between what you meant to do and what you did.
It matters because each type is prevented differently. Applying one type's fix to another type is why so many improvement plans change nothing.
Why it happens
A **slip** is an execution failure: you knew what to do and your body did something else. Putting salt in your coffee instead of sugar. The plan was right; the execution drifted.
A **lapse** is a memory failure: you forgot a step. Leaving the house without your keys, not attaching the file. Nothing was done wrong; something was not done at all.
A **mistake** is a planning failure: you did exactly what you intended, and what you intended was wrong. It is the most serious of the three, because while you are doing it everything looks correct, so nothing warns you.
A **violation** is not strictly an error: it is knowingly breaking a rule. It usually happens under time pressure or because the rule gets in the way, and once it is repeated often enough it stops feeling like a breach at all.
What it looks like in real life
Slip: the button next to it
You meant to hit "save" and hit "discard". You knew perfectly well what to do.
Lapse: the missing attachment
You write "please find attached" and send without attaching. The most universal error there is, and pure memory failure.
Mistake: the whole strategy
A business built over a year without asking a single customer. Every step was executed well. The plan was the error.
Violation: skipping the protocol
Not running the check because there is no time and "nothing ever happens". Until it does.
Signs this is happening to you
- If the failure surprises you as you are making it, it is a slip.
- If you notice afterwards that something was missing, it is a lapse.
- If on review you still think it was the right call, it is a mistake.
- If you knew the rule and chose not to follow it, it is a violation.
- If the same type of failure repeats across the whole team, the source is organisational.
How to avoid it
- Slips: separate and differentiate. These are fought with physical design: move dangerous controls away from harmless ones, change colours and shapes, make the serious action require a different gesture.
- Lapses: do not rely on memory. Checklists, automatic prompts, and systems that will not let you proceed without the missing step. The mail client that spots "attached" and warns you is exactly this.
- Mistakes: seek contradiction before executing. Since everything looks right from the inside, the only defence is someone outside. Show the plan to someone who has an incentive to find holes in it.
- Violations: look at why the rule gets broken. When everyone skips a rule, it is almost always because following it makes the job impossible on time. Punishment does not change that; redesigning the rule does.
- Classify before you propose fixes. If the failure was a lapse, more training does nothing. If it was a mistake, a checklist will not help either. Starting from the type saves months.
Real mistakes from the community
These are not examples we made up. They were written by the people who lived them.
- I moved to a new city without visiting it onceAccepted the job, signed a year lease, packed my life — all based on Google photos and one video call. The neighborhood was nothing like the pictures …
- I waited until rock bottom to try therapyYears of 'I can handle it myself' and 'it's not that bad'. It took a panic attack at work and losing a relationship to finall…
- I lost my entire thesis to one coffee spillNinety pages. One laptop. Zero backups — not even a USB stick. The coffee took four months of work in three seconds. I had to ask for an extension and…
- I co-signed a lease for an acquaintance"Just for the paperwork, nothing will ever happen." He stopped paying month six and left the country. Guess who covered the remaining 4 mont…
Frequently asked questions
What is the difference between a slip and a lapse?
A slip is doing something other than what you intended; a lapse is not doing something you intended. One is execution, the other memory.
Why are mistakes the most dangerous type?
Because they produce no sense of error while they are happening. You can execute a wrong plan for months feeling completely certain.
Is a violation the same as negligence?
Not necessarily. Most violations are "routine": shortcuts that became normal because the official procedure is impractical. Negligence implies disregard for risk and is far rarer.
Is this classification useful outside work?
Yes. Knowing whether you repeat something through carelessness, forgetfulness, or a bad plan completely changes what you should try.
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