(1) Analyse in depth an exemplar safety-critical task required of newly qualified doctors (prescribing insulin) and (2) Provide transferable insights into how undergraduate education could better educate medical students to meet the demands of practice when they become postgraduate trainees.

Document analysis of doctors’ reported experiences of insulin prescribing, an everyday task that has an emergent logic of practice and harms not just patients but (psychologically) new doctors. Application of third-generation (social emergence) complexity theory to explore why practice can be ‘mutually unsafe’.

A system of care comprising all five Northern Irish (UK) Health and Social Care Trusts, which together provide healthcare to a population of nearly two million people.

68 postgraduate year 1 and year 2 trainees (PGY1/2s), mainly PGY1s.

Thick description of new doctors’ contexts of action, reasons for acting and specific actions. We present this as a narrative compiling all 68 stories, 13 detailed exemplar stories and a diagram summarising how multiple factors interacted to make practice complex.

Situations that required PGY1/2s to act had interacting layers of complexity: (1) disease trajectories; (2) social dynamics between stakeholders and (3) contextual influences on stakeholders’ interactions. Out-of-hours working and unsuitable wards intensified troublesome contextual influences. All three individually complex layers ‘crystallised’ briefly to create ‘moments of action’. At best, PGY1/2s responded proactively, ‘stretched time’ and checked the results of their actions. At worst, PGY1/2s ‘played safe’ in unsafe ways (eg, took no action), acted on unsafe advice or defaulted to actions protecting them from criticism. Informal, pervasive rules emerged from, and perpetuated, unsafe practice.

New doctors’ work includes acting on indeterminate, emergent situations whose complexity defies rules that are determinate enough to be taught off the job. If new doctors are to perform capably in moments of action, medical students need ample, supervised, situated experience of what it is like to take responsibility in such moments.