Error Models & User Errors

Errors are part of interacting with digital systems - but they are not a sign of stupidity, they can be systematically explained. UX design must understand and intercept them.

User errors are unintended actions or outcomes during interaction. Understanding whether an error is a slip or a mistake helps UX teams prevent it through clearer design and better recovery.

Definition

A user error is an unintentional action or an incorrect result that occurs when using a system - regardless of whether the system works “correctly”.

Norman: Slips vs. Mistakes

Donald A. Norman distinguishes between two basic types of errors:

Error type	What goes wrong?	Design response
Slip	The intended action is executed incorrectly	Improve affordances, feedback and error tolerance
Mistake	The intention or underlying assumption is wrong	Clarify concepts, labels and system behavior

🟢 Briefs - “slips”

Error when executing an intention that is actually correct Example: wrong button clicked, although the destination was clear

🔴 Mistakes - “wrong decisions”

Error due to incorrect intention or assumption Example: User:in thinks “Back” means “Cancel” - but it doesn’t

Implications for Design

Slips are often caused by poor affordances or unclear feedback. Mistakes due to incorrect mental models or misleading navigation.

Reason: Human Error as a System Component

James Reason makes a distinction in his error model:

Wissensbasierte Fehler (Knowledge-based mistakes) in unknown situations → incorrect interpretation
Regelbasierte Fehler (Rule-based mistakes) known rule applied incorrectly
Skill-based slips (Skill-based slips) Familiar action automated, but incorrect (e.g. mistyping)

Design Against Errors

Principle	Implementation
Error tolerance	Undo, restore, queries before deletion actions
Feedback & status displays	What has been done? What is happening right now?
Error avoidance instead of error handling	Deactivate buttons instead of showing warning later
Consistency	Recognizable symbols, navigation, layouts
Explicit labeling	”Cancel” instead of “Back”

A practical error-prevention process

Observe where users make slips or form incorrect assumptions.
Classify the error as an execution problem or a planning problem.
Remove avoidable error opportunities before adding warnings.
Add feedback, undo and recovery for errors that remain possible.
Re-test the complete flow with representative users.

Practical Example

An e-mail system allows deletion without consultation.
→ User accidentally deletes important message.
Better: “Recycle bin” with undo function and query for sensitive content.

Conclusion

User errors do not occur because people are unfocused or incompetent - but because systems are designed to be complex, ambiguous or error-prone.
UX design should focus on being error-friendly rather than error-free.

Take Home Message

Errors are systemic - not individual. Good design makes mistakes unlikely and reversible.

Frequently asked questions (FAQ)

What are user errors in UX design?

User errors are unintentional actions or incorrect results when interacting with a system - regardless of whether the system functions technically correctly.

What types of errors does Donald Norman distinguish?

Norman distinguishes between slips (slips in the execution of a correct intention) and mistakes (wrong decisions due to incorrect assumptions or mental models).

What does James Reason's error model describe?

Reason distinguishes between knowledge-based errors, rule-based errors and skill-based errors. They show that errors can be explained systematically and are not just individual failures.

How can UX design avoid errors?

Through error tolerance (e.g. undo), clear feedback, consistent navigation, explicit labeling and the avoidance of error possibilities instead of subsequent warnings.

Why do errors often arise from the system and not from humans?

Complex or ambiguous interfaces, a lack of feedback and inconsistent structures lead to errors. Good design reduces these risks and eliminates errors.

Last modified: 17 September 2026