Tail rotor smoothing is preformed on helicopters to reduce vibrations of the airframe induced by slight imbalance of the tail rotor assembly. These vibrations are a leading cause of structural fatigue and also induce increased wear on the rotating components of the tail rotor and associated drive train. We will examine the case history of CH124 Sea King helicopter 124412, which reveals what can go wrong with a fairly basic and well documented procedure. Numerous problems were encountered such as equipment failure, incorrect assumptions, crew changes and poor inspection procedures. Success was eventually achieved and many lessons were learned through the mistakes that were made. This case history is used to train new personnel and will hopefully prevent these same errors from happening again and reduce any delays when performing tail rotor smoothing.