> The incident was triggered by a valid manual request for a squawk code. This manual request was made correctly and there was nothing abnormal or invalid about the associated flight plan.<p>> While this request was being processed, the NAS received a message for a higher priority activity to be undertaken which resulted in the squawk code allocation being paused while the system processed the higher priority message. Switching between different activities in response to prioritised requests is a normal function of the system; however, when the processing of the squawk allocation request resumed, the software defect meant it did not resume correctly and the resulting output was corrupted.<p>> The reason this scenario has not occurred before is because:<p>> 1. The defect existed in a specific subsection of code within a software module, with an exposure window estimated as approximately one millisecond.<p>> 2. For the fault to occur, a higher-priority request had to arrive during that exact millisecond while the original request was part-way through updating a value.<p>> 3. Had the higher-priority request arrived even one millisecond earlier or later, the update would have completed normally.<p>> Post-incident investigation has identified that when processing of the squawk allocation request resumed, the data associated with it had been corrupted and affected some subsequent flight data updates.