Design Principles
Slips vs. Mistakes
User errors divide into two fundamentally different types: slips (right goal, wrong action — a lapse of execution) and mistakes (wrong goal — a failure of understanding). Each requires a different design response.
Where it comes from
It's one of Don Norman's most practically useful frameworks, drawn from his work on human error. Norman split user errors into two fundamentally different kinds — slips and mistakes — each with a different cause and, crucially, a different cure.
Why it matters for your website
Norman's distinction between slips and mistakes is one of the most practically useful frameworks in all of UX. A slip happens when a user knows what they want to do and intends the right thing, but executes incorrectly — usually due to inattention, habit, or poorly placed controls. A mistake happens when the user's mental model of the system is wrong, so they form the wrong goal in the first place. The remediation is completely different: slips are prevented by constraints, confirmation steps, and separation of dangerous actions from safe ones; mistakes are prevented by clearer feedback, better conceptual models, and design that corrects misunderstanding rather than punishing it. Diagnosing which type of error a design is inviting determines which fix is needed.
The distinction is diagnostic. A *slip* is a lapse of execution — the user intended the right thing but did it wrong, usually through inattention or habit. A *mistake* is a failure of understanding — the user formed the wrong goal because their model of the system was wrong.
Why it matters is that the two demand opposite fixes. Slips are prevented by constraints, confirmations, and keeping dangerous actions away from safe ones; mistakes are prevented by clearer feedback and better conceptual models that correct the misunderstanding. Apply the wrong remedy and the error persists.
Wrong vs right
Treating a slip (a user accidentally deleting the wrong item) as if it were a misunderstanding, and adding a tutorial.
Preventing the slip with a constraint or confirmation — an undo, or separating the delete control from common ones.
Treating a mistake (a user with the wrong mental model) by adding a confirmation, which doesn't fix the misunderstanding.
Correcting the mistake with clearer feedback and a better conceptual model, so the right goal forms.
Punishing users for errors without diagnosing whether they're slips or mistakes.
Diagnosing the error type first, then applying the matching fix.
Understanding Slips vs. Mistakes
Norman divided user errors into two fundamentally different categories. A slip happens when a user has the right intention but executes it incorrectly — the goal was correct, but a lapse of attention, a habit, or a poorly placed control produced the wrong action. A mistake happens at a deeper level: the user's mental model of the system is wrong, so they form the wrong goal in the first place.
The reason the distinction is so practically useful is that the two errors need opposite remedies. Slips are prevented by constraints, confirmation steps, undo, and separating dangerous actions from routine ones — guarding the execution. Mistakes are prevented by clearer feedback, better conceptual models, and design that corrects the misunderstanding — repairing the user's understanding.
Diagnosing which type of error a design invites is what determines the right fix. A confirmation dialog won't cure a mistake born of a wrong mental model, and a clearer explanation won't stop a slip caused by a mis-tap — matching the remedy to the error type is the whole point. It connects to constraints, error prevention, and the gulfs of execution and evaluation.
How Kweri checks it
Kweri can flag design conditions associated with each error type — controls placed so a slip is likely (dangerous actions next to routine ones, no confirmation or undo), or interfaces whose model seems likely to mislead users into the wrong goal. What it can't always determine without observing real users is which errors people actually make and why. So Kweri surfaces likely slip and mistake risks and prompts the matching remedy — constraints for slips, clearer models for mistakes — while diagnosing the real errors your users make is a job for testing.
FAQ
What's the difference between a slip and a mistake?
A slip is when a user intends the right thing but executes it wrong — a lapse of action, often from inattention or habit. A mistake is when the user's mental model is wrong, so they form the wrong goal in the first place. They have different causes and different fixes.
How do you fix slips?
Slips are prevented at the execution level: constraints, confirmation steps, undo, and separating dangerous actions from routine ones. Since the user's intention was right, the goal is to stop the right intention being carried out wrongly.
How do you fix mistakes?
Mistakes are fixed by repairing understanding: clearer feedback, a better conceptual model, and design that corrects the misunderstanding so the user forms the right goal. A confirmation won't help, because the user's intended goal was wrong to begin with.
Why does the slip-versus-mistake distinction matter?
Because the two need opposite remedies. Applying a slip fix to a mistake — or vice versa — leaves the error in place. Diagnosing which type you're dealing with is what tells you which fix will actually work.
Who developed the slips and mistakes framework?
Don Norman, as part of his work on human error in The Design of Everyday Things. It's one of his most practically useful frameworks for diagnosing and preventing user errors.
Related principles
Stop problems before they happen — that beats even the best error message.
Error messages should be plain English, name the exact problem, and suggest the fix.
Errors should be flagged as users complete each field, not after the entire form is submitted — post-submit error discovery forces users to stop, hunt for problems, and often repeat work they've already done.
Attribution & sources
Identified by Don Norman. Catalogued from Nielsen Norman Group — Slips and Mistakes.
Norman's framework for human error; the linked article is the reference used here.
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