Not doing your homework. Knowledge is power. Tremendous information is available on the Internet. There is no excuse for entering the market unprepared.
Trying to make a shrewd investment. People need to buy based on what fits their family. Don't try to guess what will happen to the market.
Choosing a poor location. Even within a neighborhood, location matters. Is it on the busiest street? Is there a shopping center out the back window?
Overlooking an inferior floor plan for an attractive exterior. It may have gorgeous curb appeal, but you don't live on the lawn. No matter how attractive the exterior, you need a livable home.
Overlooking how the house will function for your family. How do you really live? Do you really need a formal dining room and living room? Would you be happier with an eat-in kitchen and a great room and a den to use as a home office? The house only needs to fit one family -- yours.
Not having the home properly inspected in a resale. This is not the time for surprises. Get an inspection from a qualified, respected professional.
Not checking out the builder's reputation on a new home. Talk to three or four people who live in the builder's homes and see what they have to say. If one builder did all the houses in a neighborhood, talk to the residents and get their input. (It's also a great way to see what your neighbors would be like.)
Not getting what you want because you're impatient. This is a big decision. You need time. Impatient decisions can lead to mistakes.
Waiting for a better market and interest rates. Warren Buffett says the rear view mirror is always clearer than the windshield.
Not buying at all. If you can afford a home and you don't make that purchase, you'll lose the benefit of tax deductions, building home equity and the appreciation in value.