If you don't have a fixed-price contract, you'll feel it in the pocketbook. And then there's the change order. First, it's difficult to tell how something is going to look and feel from the blueprints. This is true for long-time builders as well. Sometimes, the only way to figure out a drawing doesn't work is to build it and see what's wrong.
Architects make mistakes, too.
Although architects are trained to visualize how a house is going to look once it is built, and while there are computer assisted design programs to aid the process, it all boils down to imagination. Sometimes it's difficult for the architect to catch every design flaw in a drawing. The architect may also not be familiar with your town's local building code. If his design conflicts with local building code, you'll want your builder to catch it, so your house ends up passing inspection. Finally, you may have a change or mind, or heart.