Northbeach, I can confirm one practice which is listed under "mechanical breakdown". Some years ago I was among 200 Monarch passengers stranded at Faro in Portugal, "our" plane was delayed through a technical fault. It later emerged that in fact there was nothing wrong with "our" aircraft at all. Another plane went technical at Malaga. This second aircraft was due to fly back to Gatwick and then to Goa. "Our" plane was substituted for this and sent to Goa. The broken down aircraft was apparently not easily repaired, so we poor SH passengers had to wait until Monarch had another plane to send out for us.
In the subsequent correspondence the airline were very interested in how I found out what had happened. Without divulging the source of my information, I asked them to confirm the truth of it, which they did somwehat reluctantly saying it is not their policy to discuss aircraft movements with passengers.
Ironically, our flight out the previous week had been performed by an ancient Tristar, wet leased from a (then) well known Icelandic airline to cover an aircraft unavailability. The cabin staff had told us that they had been given just three hours to get from stand-by in Reykjavik to Birmingham for our scheduled departure time, (they didn't make it - quite). Given that we had to cool our heels in Faro for over twelve hours, I asked Monarch why they had not engaged the same company the following weekend. It was at that point they stopped communicating with me, I never heard any more from them -funny that isn't it?
BTW, How did I know about the aircraft movements? - simple, the pilot of the plane who eventually picked us up told us - I wonder if he was admonished?