‘Human factors’ is an influential rationale in the UK national health service to understand mistakes, risk and safety. Although there have been studies examining its implications in workplaces, there has been little investigation of how it is taught, as a form of professional morality. This article draws on an observational study of human factors teaching in four hospital simulation centres in London, UK. Its main argument is that the teaching of human factors is realised through an identification and positive evaluation of ‘non-technical skills’ and the espousal of ‘non-judgemental’ pedagogy, both of which mean that mistakes cannot be made. Professional solidarity is then maintained on the absence of mistakes. We raise questions about the ethics of this teaching. The study is situated within a history of ethnographic accounts of medical mistakes, to explore the relationship between claims to professional knowledge and claims about failure.