This paper presents a case history outlining the investigation performed to evaluate the causes of failure between a C77 block and tubing bonnet line, used for delivering scale inhibitor to a production well. The data review indicated that the failure occurred as a result of synergistic corrosion effects, including incompatibility between the scale inhibitor chemical and the metallurgies of the block (13Cr SS) and tubing (carbon steel). The analysis showed that while each of the relevant corrosion mechanisms (oxygen corrosion, galvanic and localised corrosion) may not be the root cause for the failure individually, in combination they proved detrimental to system integrity. The study also highlighted that while chemical inhibitor compatibility testing with appropriate flow-wetted metallurgies is common practice, the environment and configuration used for testing is not always representative of the field conditions or specific process operations (e.g., oxygen ingress due to unblanketed tanks, presence of dissimilar materials). In the present case, for example, the most significant oxygen sources were found to be the dissolved oxygen in the tap water used to mix the scale inhibitor to make up the final formulation; and the glycol blended into the product to give it ‘winterised’ properties. Lastly, the paper summarises the recommendations that were put forth to address the issue, such as evaluation of different metallurgies, a more suitable scale inhibitor and implementation of process monitoring, especially of oxygen levels.