Errors split by cognitive stage: slips and lapses are failures of execution where the action deviates from intention (attentional or memory failures), while mistakes are failures of planning where the intention itself is wrong even if executed perfectly. Because they arise at different stages, they call for different countermeasures: better monitoring for slips, better knowledge or reasoning for mistakes.
Human performance operates at three levels: skill-based (automatic, routine actions), rule-based (applying stored if-then rules to familiar problems), and knowledge-based (slow, effortful reasoning in novel situations). Errors take characteristic forms at each level: inattention slips at the skill level, misapplied or bad rules at the rule level, and incomplete or biased reasoning at the knowledge level.
Accidents in complex systems combine active failures (unsafe acts by frontline operators with immediate effects) with latent conditions (decisions by designers and managers that lie dormant in the system until combined with local triggers). Latent conditions can be identified and remedied before an accident, making them a more productive target for prevention than the sharp-end operators.
Hazards reach victims only when they pass through multiple successive defensive layers, each of which has holes caused by active failures and latent conditions. An accident happens when the holes in many layers momentarily line up to form a continuous trajectory; because the holes shift, the alignment is rare but not impossible.
The same cognitive mechanisms that make human thought powerful also generate its systematic errors: the mind retrieves stored patterns by similarity and frequency, so under uncertainty it defaults to familiar, frequently used responses. Errors are therefore not random breakdowns but the predictable price of an efficient, pattern-matching cognitive system.
Recurring error types are shaped more by the task, tools, and situation than by the individual's carelessness or competence, so different people in the same error-provoking conditions produce the same mistakes. This shifts blame from punishing individuals toward redesigning the conditions that reliably elicit the error.