What assignment has haunted John for nearly twenty years?

Early in the novel, the reader learns that Control has had several assignments over the years, but that one job one wrong has haunted him for the last two decades. He allowed himself to become emotionally involved with one of the people he was supposed to be gathering intelligence from. As a result she was brutally killed. He was implicated and reprimanded by Central, then sent to another assignment. However, in each subsequent assignment he was scrutinized so heavily that invariably something was found to be wrong.