Relying on word of mouth
I hear it all the time.
I get my business through word of mouth, says the business owner proudly.
This has to be the most dangerous way to run a business because you are very vulnerable to the attack of a competitor. 
How can I explain this to you when you don't really understand how to create POWERFUL MARKETING?  Oh, my goodness.  Well, I'll try anyway.  You probably won't get it.  Only after I've worked with you might you get it.
First of all, you have no control over how your marketplace sees you. 
You're depending on others to tell the marketplace who you are and why to buy from you.  That can change in an instant and you have no control over it and your business will dry up.
Second, you have no MARKETING POSITION.  
That is to say that you have not told the marketplace 'why you'.  Why should anyone buy from you as opposed to the competitor?  Now don't give me that nonsense mantra that you are giving 'good service'.  If that's what you think then read the item on 'good service'. 
What you are doing with word of mouth is you are expecting others to spread the word but you don't know what they are saying and you don't have any control on what they are saying.  And you don't know when they are saying it. 
So here you sit hoping to make a sale through word of mouth when you haven't a clue as to what or when they are saying anything about you.
Third, it is easy to steal your market since you are not actively telling the world 'why you'.
I could go on and on but the above should be enough to show you why expecting your business to continue because of 'word of mouth' marketing is dangerous.