There is an ongoing debate regarding the cause of diagnostic errors. One view is that errors result from unconscious application of cognitive heuristics; the alternative is that errors are a consequence of knowledge deficits. The objective of this study was to compare the effectiveness of checklists that (a) identify and address cognitive biases or (b) promote knowledge retrieval, as a means to reduce errors in ECG interpretation. Novice postgraduate year (PGY) 1 emergency medicine and internal medicine residents (n = 40) and experienced cardiology fellows (PGY 4-6) (n = 21) were randomly allocated to three conditions: a debiasing checklist, a content (knowledge) checklist, or control (no checklist) to be used while interpreting 20 ECGs. Half of the ECGs were deliberately engineered to predispose to bias. Diagnostic performance under either checklist intervention was not significantly better than the control. As expected, more errors occurred when cases were designed to induce bias (F = 96.9, p < 0.0001). There was no significant interaction between the instructional condition and level of learner. Checklists attempting to help learners identify cognitive bias or mobilize domain-specific knowledge did not have an overall effect in reducing diagnostic errors in ECG interpretation, although they may help novices. Even when cognitive biases are deliberately inserted in cases, cognitive debiasing checklists did not improve participants' performance.