1. They Don't Get Educated.  They are so excited to see what is out there they immediately go to the internet and start looking at homes to see what they can buy.   Although they do need to see what's available at some point,  the danger is they will see something that looks good so they then want to view it asap before they actually get the education needed to make a good decision.   

2. They don't interview an agent before looking at homes.   This is obviously a big decision so you must ensure the person that will assist you with that decision knows what they are doing.   The agent should be able to demonstrate their knowledge and explain the process to you.   Ask lots of questions about their experience.

3. They are not properly approved for financing.  Unless you have submitted your proof of income and provided permission for a credit check to a Mortgage Professional or a bank you are not pre approved.

4.  Unrealistic Market Expectations.   No matter what price range you are in you can't get everything you want, so its important to be clear on the must haves and the ones you can live without.

5. Can't make a reasonably fast decision.  When buyers find a property they really like they quite often have a conundrum; they want to go home for 3 days and think about it, but they don't want anyone else to buy it while they think.   As silly as that sounds, I often get asked the question,  "Do you think it will sell fast"?   The truth is,  I have no idea but I will say this "if you like it, chances are high someone else will too".