and you can 'swiss cheese' your barriers, train over and over again, create policies and procedures, revise checklists, encourage everyone from the PIC to the plane spotter at the fence to point out errors and... still accidents are going to happen.
I have posted here on this before. Allow me to repeat:
All accidents are avoidable (barring 'acts of God' or the enemy); if you think they are not you are on the way to becoming a statstic'.
Forget the 'swiss cheese' analogy. It is not acceptable. Get yourself some Cheddar!