A pilot for KLM airlines suffered a heart attack as the plane he was piloting taxied the runway. 

Pilots in cockpit
A pilot for KLM airlines suffered a heart attack as the plane he was piloting taxied the runway. 
 
The incident took place at Glasgow International airport where the Dutch captain was resuscitated on board by airline crew with the help of a passenger. The flight was headed for Amsterdam. 
 
Fire crew from the airport helped take the pilot off of the plane, where he was met by an ambulance crew and took to the hospital. 
 
It was reported that the pilot was in a stable condition. 
 
The flight had to be cancelled and all passengers were booked on to the next available flight. 
 
An airline pilot suffering a heart attack constitutes an extraordinary circumstance, and therefore, this is an example of a flight being delayed by three or more hours or cancelled altogether when passengers are not entitled to flight delay compensation. 
 
On a lighter note, another medical emergency caused a flight delay but it happened mid-air. A passenger on a US airline gave birth mid-flight.