What is one example of a special condition when ATC services may not follow first come first served?

Military air evacuation requests are a critical situation that can necessitate prioritization over standard procedures like first come, first served. In such cases, the urgency and importance of the evacuation mission warrant immediate attention from air traffic control. These operations often involve the transit of medical personnel or patients who are in dire need of urgent care, particularly during military conflicts or humanitarian operations.

Due to the nature of these missions, ATC may provide priority handling to ensure that the affected parties receive timely assistance. This can involve expediting the aircraft's departure, providing clear flight paths, or facilitating quicker landing times. The importance of the mission and the potential life-saving implications justify this deviation from standard procedural norms.

Other options like aircraft that are behind schedule, flight leisure requests, or routine maintenance operations do not typically carry the same level of urgency and therefore do not warrant deviation from the first come, first served principle.