Ensuring the competence of healthcare professionals’ is core to undergraduate and post-graduate education. Undergraduate pharmacy students and pre-registration graduates are required to demonstrate competence at dispensing and accuracy checking medicines. However, competence differs from understanding. This study determined the competence and understanding of undergraduate students and pharmacists at accuracy checking dispensed medicines. Third year undergraduate pharmacy students and first year post-graduate diploma pharmacists participated in the study, which involved an accuracy checking task and concept mapping exercise. Participants accuracy checked eight medicines which contained 13 dispensing errors and then constructed a concept map illustrating their understanding of the accuracy checking process. The error detection rates and types of dispensing errors detected by undergraduates and pharmacists were compared using Mann–Whitney and chi-square, respectively. Statistical significance was p ≤ 0.05. Concept maps were qualitatively analysed to identify structural typologies. Forty-one undergraduates and 78 pharmacists participated in the study. Pharmacists detected significantly more dispensing errors (85%) compared to the undergraduates (77%, p ≤ 0.001). Only one undergraduate and seven pharmacists detected all dispensing errors. The majority of concept maps were chains (undergraduates = 46%, n = 19; pharmacists = 45%, n = 35) and spokes (undergraduates = 54%, n = 22; pharmacists = 54%, n = 42) indicating surface learning. One pharmacist, who detected all dispensing errors in the accuracy checking exercise, created a networked map characteristic of deep learning. Undergraduate students and pharmacists demonstrated a degree of operational competence at detecting dispensing errors without fully understanding the accuracy checking process. Accuracy checking training should be improved at undergraduate and post-graduate level so that pharmacists are equipped with the knowledge and understanding to accurately check medicines and detect dispensing errors, thereby safeguarding patient safety.