Mistake #1:
Not knowing how much they can afford before they make an offer.
Call a lender to get Pre-quailified before you ever start to look for a home. This will let you know how much you have to purchase with. 

Mistake #2:
Not realizing in advance who the real estate agent represents.
Make sure your Realtor discusses Agency Relationship with you. 

Mistake #3:
Not realizing that the wrong mortgage can cost thousands of dollars in needless interest and taxes.
Check with an accountant about what the long-term effects will be on your income.

Mistake #4:
Not discovering hidden defects before they buy a home.
One of the most expensive mistakes is also one of the easiest to avoid, by having a professional pre-purchase home inspection.

Mistake #5:
Not knowing how much credit can affect their ability to buy or refinance a home.
Before you buy a home, many of the clouds on your credit history can be cleared up or even eliminated. Your mortgage professional can help you review and prepare your credit file in advance.