That the newborn and toddler stages are a doddle in comparison to the teenage years.
That parenting doesn't end. Ever. Even when they are adults they still need you. Financially, emotionally.
That no two children are the same, that if your first sleeps through the night, there's no guarantee the next one will.
That saying 'I'm never going to ... (add in 'your rules') is pointless. Because you probably will.
That despite all your parenting techniques and your determination that your child will be a certain way, they are actually individual human beings with their own personalities and traits and it doesn't matter how much you might want them to be an athletic type, they may prefer books.
That no matter what we do, at some point we are doing it wrong (according to your child) and on those days you just think 'fuck it' and reach for the wine.