I was thinking about this after the most recent fight thread about SAHMs/WOHMs where one poster said something along the lines of a parent could stay at home until their DC was 18 and they could still turn out to be a dick.
I remember someone on a previous thread talking about the '3 P's' -personality, parenting and peers and I really agree with that. I think you can be an amazing parent but, if you have a child with a difficult temperant who then gets in with a 'bad crowd' as a teenager, they can go massively off the rails despite your best efforts.
Your ability to control what your DCs do diminishes rapidly as they get older especially when they reach secondary school age. We probably all know of families who have parented their DCs similarly and they turn out very differently.
It seems that the majority of people who have had poor parenting will have some degree of difficulty as an adult but that even 'perfect' parenting won't guarantee that your DCs turn out OK. Which is a bit depressing.