1. Not taking into account costs of home ownership in their budgeting process. They get preapproved for x and think they can afford that amount. But you should plan on buying a house under what the lender approves you for, so you are not house poor later. Things happen -- maintenance items such as roofs leak, furnaces break down -- and you need to have a cushion so you're not stretched too tight when bad things happen at the house.
2. In negotiating for the house purchase, not understanding the long term cost of an extra $5000 or $10,000. When negotiating, frequently two parties are almost there, and only a few thousand dollars may separate the buyer and seller. Buyers need to ask themselves, "How badly do I want this house?" If you're $10,000 apart, that's an additional $49.19 per month on a 30 year fixed loan at 4.25%. And $5000 is only $25 extra a month So if your answer is you have to have this house and you'll cry if you don't get it, can you afford an extra $49/month? That's maybe one dinner out for a couple, or giving up one fancy latte for 10 days each month. Can you do that? If so, why not pay the extra $10,000? If you don't care if you lose the house, then fine. Let it go. And if you cannot afford the extra $49/month then perhaps you're already over your head right now and should be looking at a less expensive home. I did this with my own home, which I bought 9 years ago. We were $10,000 apart and I really wanted this house. The extra $50/month has brought us 9 years of happiness in this house, which my family loves. I never looked back.
3. Not getting a home inspection. Some buyers shave costs by skipping the home inspection. Instead they may have Uncle Larry look at the house, or trust dad's instincts. But unless Uncle Larry or dad are certified home inspectors, you're taking a chance. The $350-500 you put out now for a home inspection will seem like small potatoes if the home inspector finds a big problem you would regret later. Get a home inspection, or take your chances and don't complain later if the house has unforseen issues. Skipping this step is being penny wise but pound foolish.