Guidelines support clinicians in safe decision making for a range of complex tasks but guidelines may also lead to patient harm. This study used medication errors reported to the National Reporting and Learning System for England and Wales to identify errors related to guideline use. Errors included inability to locate guidelines (e.g., website was down), not finding the relevant information within the guidelines (e.g., dosing for pregnant patients), and incorrect use of the information (e.g., disagreement between staff around meaning of information). Local, as opposed to national or organizational, guidelines were implicated in most errors and thus require further research.