Textbook of Usability

A slip is an error of execution — the user intended the correct action but performed it incorrectly. Slips occur at the level of action, not planning: the user knew what to do but slipped up in doing it. Pressing the wrong key, clicking the adjacent button, or selecting the wrong item from a drop-down list are all slips.

James Reason (1990) distinguishes slips from lapses (memory failures) and mistakes (planning failures). Each error type has different causes and requires different design mitigations.

Slips are typically caused by:

• Attentional capture — attention drawn elsewhere at the critical moment
• Motor imprecision — missing a small target, mis-hitting a key
• Similarity — confusing visually or spatially adjacent items
• Automation — well-practised sequences running when inappropriate

Design mitigations for slips focus on the physical interface:

• Larger targets (Fitts's Law) to reduce motor imprecision
• Spatial separation of actions with very different consequences ("Delete" far from "Save")
• Confirmation for irreversible or destructive actions
• Shape/colour differentiation of visually similar items
• Undo to allow recovery when slips happen anyway

Slips are the easiest error type to prevent through good design. They are also the hardest for individual users to prevent through effort — better design is more effective than more training.