Mahaim fibres are cardiac accessory bundles which can cause dangerous tachyarrhythmias. Despite their first description being many years ago, their recognition and diagnosis still proves difficult. The electrocardiographic features of Mahaim pathways are subtle and shared with other accessory pathways; as such, an electrophysiology study must be used to diagnose this pathway. Even then, it is often a process of elimination which leads to the eventual diagnosis. Here, I will present the case of a young woman with a peculiar history who was ultimately diagnosed with a Mahaim fibre pathway. Using this case, I will describe the key electrophysiological features and the treatment of the Mahaim fibre pathway.