This article by Paul Aylin and colleagues contains errors that escaped the notice of both the BMJ and the authors during the editorial process. The first sentence of the opening box should read: “It has been suggested that an estimated 850 000 medical errors occur in NHS hospitals every year resulting in 40 000 deaths.” In the “basic figures” section, the first sentence should read: “On average 2.2% of all episodes (about 275 000 [not 27 500] per year) included a code for an adverse event.” And we initially posted an incomplete version of table A on bmj.com. It has now been updated to provide full ICD-10 codes. We apologise for our lapses.