One of the most common mistakes buyers can make is not getting themselves pre-approved by a lender. Imagine spending weeks looking at homes worth $250,000 only to to find out, when push comes to shove, your debt to income ratio will only support a home up to $200,000.
You might not think that is a big deal, but in many markets there is a tremendous difference in homes just $50k apart. It could force you to look in areas you were not prepared to look in. The price difference could change the number of bedrooms, lose you the upgraded kitchen, in a nutshell it changes your expectations for the worst and you walk away deflated and beaten by the market.