Textbook of Usability

A lapse is an error of memory — the user forgot a step, lost track of their position in a procedure, or failed to monitor a value. Unlike a slip (execution error) or a mistake (planning error), a lapse involves the user temporarily losing information from working memory.

Common lapses include:

• Forgetting to perform a step in a multi-step procedure
• Losing track of how many items have been entered
• Failing to return to an interrupted task
• Forgetting that a process is still running
• Missing an item on a checklist

Lapses are especially common under time pressure, fatigue, interruption, and cognitive overload — conditions that stress working memory. A nurse interrupted mid-way through drawing up a medication may lapse on whether they've already added a dilutant.

Design mitigations externalise memory requirements:

• Checklists that display each step for verification
• Progress indicators showing what's been done and what remains
• Auto-save to preserve state across interruptions
• Persistent breadcrumbs showing position in a hierarchy
• Confirmation messages that summarise what's about to happen

Lapses are the reason the aviation industry adopted checklists and why the WHO Surgical Safety Checklist reduced complications by 36% in a multinational trial. No amount of expertise eliminates the working memory limitations that cause lapses — good design externalises the remembering.