Once there are too many navigation menus to accommodate in a single line, the overflowing menus move onto the next line rather than creating a horizontal scroll bar for them. This above behaviour is correct and great but the css styling isn't quite supportive to it.
There is no background (url("../images/admin-tools.png") repeat-x scroll 0 -205px transparent) present for the next line of menus and onwards.
<ul id="navigation-menu"> this element should have a property 'overflow: auto' because otherwise the menu overlaps over its next (parent's)sibling element for example <div class="breadcrumbs">
so a simple and quick fix to this would be to add 'overflow: auto; height: auto;' css properties to <ul id="navigation-menu"> and 'position: static;' to its child li's