A key is to have a navigation bar that flows from broad to narrow, or in other terms, have it show the actual levels. Taking the navigation bar above, if a visitor is sent to the clarinet section of the site and wants to locate the trumpets, it must be clear which section to revert to. Drums People would have a difficult grasping the overall structure of the website and it would be unclear which link to go back to in order to find the trumpets. Clarinets is not a subcategory of Flutes, and Drums is not a subcategory of Clarinets. The user might then go back to the start page and begin looking for the trumpets from there, or just give up.
Problems can arise when a link takes the user from one type of navigation bar to another.
But most users would not see that and just end up ignoring the navigation bar because it leads to more confusion. A good site designer would think through these issues with the hope that a navigation bar would be beneficial to the surfing experience on their site. From informal questioning, most people actually do not look at those link bars. They feel more comfortable just going back to the homepage and working their way through the site. So to those people, the bar is just a waste of valuable screen real estate.