A term we use in ATC incidents quite a lot is " The Swiss Cheese" effect. Where a number of small incidents not catastrophic on their own but when all lined up with the holes in the cheese create an incident.......trying to take on board everything that has been spoken about on this forum so far I feel the outcome of this incident will be the same.....one small problem alone not catastrophic but 2 or 3 all lining up with the holes and the end result sadly what we saw here!