Some mistakes, like yelling, screaming and hitting are obvious and can cause damage even if used only once. 

Other mistakes don't do much damage if only done once, but when used over and over, they can chip away at that benefits of the good you are doing. These mistakes are less obvious.

Then there are mistakes a lot of parents make that are completely unintentional, such as:
- Intentionally doing something you think you are supposed to do, but that might actually be harmful (to varying degrees).
- Intentionally doing something that "works," in the short-run, but that research has shown has negative long-term results that create bigger problems they the little problems they first solved.
- Unintentionally doing something because you don't know any better, lacking the knowledge or skill to do better. You would do better if you knew better!

Thing is, most parents do these things without ever knowing that what they are doing is ineffective --- until it's too late and the damage is done.

By then, they are seeing problematic behavior issues that most parents think are "common," "normal," and "expected"! 

Without these mistakes and with effective parenting skills, parenting can actually be a pretty smooth process! Not that you don't hit bumps, but you have the knowledge and skills to prevent and resolve the problems that do arise.