Slips and Mistakes

Two kinds of human error. A slip is doing the wrong action while intending the right one, an execution failure usually caused by inattention or habit. A mistake is forming the wrong intention or plan, a knowledge or rule failure. The two need different design remedies.

Why it Matters

Norman reframes error as a design failure, not a personal failing: people will err, so systems should prevent, tolerate, and allow recovery from errors. Treating errors as "user stupidity" guarantees they recur.

Signals

• Experienced users making "careless" slips.
• Well-meaning users making the wrong decision from a flawed model (a mistake); a blame culture around error.

Benefits

Designing for error reduces harm, builds trust, and surfaces the real, structural causes instead of stopping at blame.

Risks

Blaming the operator and stopping there; designing only against mistakes (knowledge) while ignoring slips (attention), or the reverse.

Tensions

Preventing every error can over-constrain capable users, and warnings fatigue until they are ignored. The balance is sensible defaults, confirmation for the irreversible, and easy undo.

Examples

Typing into the wrong field is a slip; choosing the wrong treatment from a wrong diagnosis is a mistake; undo and confirmations are error-tolerant design.