Being a parent isn't easy and it comes with a lot of responsibility. You can pick up one of thousands of books on parenting and still never fully understanding everything that comes with being a parent.
As a parent myself here is my perspective of some of the top mistakes parents make with kids.
Parents these days lead busy work and home lives which can be overwhelming and often time what gets pushed to the back burner is consistency. If you have a set or process for rules, disciplines, rewards, activities etc it is vastly important that you are consistent with implementing and meeting them.
The second your consistency becomes inconsistent kids will notice and it can have negative affects on both their behavior as well as on their respect for you as an authority. This in the long run is detrimental to helping raise well rounded and responsible children. So take those extra steps and time each day to show consistency.
I believe "Consistency is comforting for both parent and child" whether it be with rewards or responsible discipline.
Though there will be days when keeping a routine is simply not a possibility but overall it is beneficial for you or your kids to have some sense of routine in your day to day lives (this goes hand in hand with consistency). Implement a: school, work, dinner, cleaning, playtime and bedtime routine. This will set boundaries for the kids so that life does not feel chaotic and they are not left to "figure out what to do with their days". This can help combat some behavioral issues as well as reduce how often you are needed to "redirect or direct" them to keep them occupied, productive or out of trouble.
In this day and age a lot of parents have come to rely on the "Do as I Say and Not as I Do" philosophy. Well in text it sounds fine and dandy but in real life that is not how things work in family life.
This is a "Practice what you Preach" and "Lead by Example" kind of world. Children are taught through actions of those in their lives so consider exactly what you are teaching your children.
If you take part in something that you wouldn't want your child to then don't make excuses for it - quit it. Show them through example what it takes to make it in life and what it is to be a good person. Simply through them seeing you make a positive change and knowing you do it for all the "right" reasons can be a healthy experience for your kids.
You may just be surprised at how well and how quickly they learn that lesson.
Not Being Accessible/Approachable
I feel one of the biggest parenting mistakes is to not be accessible and approachable to your children. Not being accessible/approachable doesn't mean that you are locked up in your room or that you are unable to be gotten to physically. It means not making yourself accessible to them emotionally.
When a kid gets in trouble or faces something in their lives that is difficult (drugs, pregnancy, bullying, depression etc) they should feel that they can come to you. After all you are the ones in their lives that they should feel they can rely on, at all times.
This does not mean they don't have consequences for their actions (where applicable) but it means they know that they can come to you and talk to you and you will listen and help them through this tough situation. And any consequences that come about from such a situation should be handled in a way that the child understands - which typically entails laying it all out - in a controlled way and making sure they understand their actions that brought about the consequences.
So make sure your relationship, as a parent, with your children is one that is full of support, accessibility and guidance.
It will never be beneficial to tell a child all the things they could have been, done or should have been/done or will be.
There is something to be said for acknowledging the positive and individual choices that a child makes (when it is a choice that is not negative), regardless of whether it is what you expected. A child needs to know that as a parent you respect and support them as their own individual person.
As a parent you will always want the best for your child in: love, careers, emotions etc. And there is nothing wrong with that. I believe you need to urge them to be all they can be without turning your hopes into an expectation of them. That can lead to serious emotional shortfalls in the future, rebellion and rarely leads a child to happiness.