Teaching or helping your client with a task that you're unqualified to teach.
I would never hire a coach to help me reach a place they have never been. In my practice I think it's even unethical to work with a client that wants to go somewhere I have never been. I point those people in another direction where they will get the service they need.
Nothing prepares you to help others better than having lived something out in your own life. As I always say, find someone at the level you want to be at and learn from them.
Would you take financial advice from someone who has been broke all of their life and never seems to be able to get it together? What about weight-loss advice from your friend who never leaves the couch and weighs 300 pounds? Not a chance!
I don't want investment advice from someone fresh out of school with no money in the market. I am sure they have a certain amount of perspective they can add to to my investments, but I want the best, and I'm willing to pay for it.
For the same reason, if your business is a marriage counselor, but you have been married three times, I am guessing you are not making much money. If you offer auto repair but you have to constantly bum rides to work because your car is on blocks, there is a problem.
Find something you have experience in and excel at doing. Can you shape a business around that? I promise you the money will be there.