ECT.
It does not matter how well maintained a fleet is, you are still at times going to end up with minor defects - half way through the day someone may check a landing light and the bulb blows, or the pitot heat burns out etc. etc. Whilst it may be ideal to get these sorted straight away, even the simplest job will take the aircraft out of service for half an hour or so - not ideal on a fully booked sunny Sunday even if you have the engineers and parts available, which is often not the case. Some defects are inevitable at times and you need to know what you can and can't accept when this happens.