It's All About the Price - A lot goes into buying a house than just the price. There may be hidden costs for maintenance and care or the seller may be interested in other things you could offer other than price.
Getting Too Hung Up On a Price - A couple thousand dollars difference in the price, may not make as big of a difference as you think. It doesn't actually change your payment too much and you may be missing out if you get focused on exact amounts rather than the whole picture.
Fall in Love Too Fast - If you see a home and fall in love, don't let everyone know. Keep quiet in the home as you look as well. You never know who may be listening and this may hurt your negotiation ability down the road.
Freaking Out Over Findings - Home inspections are important, however, every home will have something wrong with it. Don't freak out! Give the seller a chance to make it right! Some things may be really minor and easy to fix. You don't want to blow up the deal because of fear.
Fighting Over Small Repairs - After an inspection, there may be several things that need to be fixed. Don't fight over the small things. Some things don't take much and you could lose out on a home that is a great price (and great for you) for something small. It's the big things that need to be taken care of up front.
Discussing Decor - Decor can (and will) be changed when the seller moves out. Even if you can't stand the decor, pay attention to the home's condition and the condition of the property. Decor is a cosmetic and easy fix and doesn't play a huge roll in negotiating. Don't think you can undercut a seller because you hate how they decorate.