Insidious incapacitation is considered to be the most dangerous form of incapacitation as it 'sneaks up on you'. If you had an explosive decompression the onset of hypoxia and accompanying incapacitation would be very obvious and therefore, hopefully something would be done about it. However if there was a slow decompression it is possible that things could go unnoticed with a resulting insidious onset of hypoxia/incapacitation and no action would be taken......
Not to be confused with another one straight from the question bank; What is the most common form of pilot incapacitation where the answer is Gastroenteritis.