By far the most common problem is the fear parents have that they'll do the wrong thing. They'll buy more books, or go to more lectures, or read columns about child-rearing, and are very easily intimidated by these things. Half the articles I've written say don't be afraid of doing the wrong thing. There are a minority of parents who are still afraid to do what should come naturally. They've thrown away their natural self-assurance.
Babies learn very soon that the mother has less conviction than the baby has.
This is why there are so many pesky children who are arguing with their parents all the time. It's wrong for children to be brought up thinking they can bully and shame their parents into silence. But it's also hard on parents. I say parents should give their children leadership, firm but loving leadership. I think that parents should ask for respect from their children.