Twitter Bootstrap: dropdown

strange behaviour

by A B

HTML

<link rel="stylesheet" href="http://twitter.github.io/bootstrap/assets/css/bootstrap.css">
<link rel="stylesheet" href="http://twitter.github.io/bootstrap/assets/css/bootstrap-responsive.css">
<script src="http://twitter.github.io/bootstrap/assets/js/bootstrap.js"></script>
<div class="navbar">
    <div class="navbar-inner"> <a class="brand" href="#">Title</a>

        <ul class="nav">
            <li class="active"><a href="#">Home</a>
            </li>
            <li><a href="#">Link</a>
            </li>
            <li><a href="#">Link</a>
            </li>
            <li><a class="btn btn-small btn-info" data-toggle="collapse" href="#AdvancedSearch">Search</a>
            </li>
            <li>
                <button type="button" class="btn btn-small btn-info">Search</button>
            </li>
        </ul>
    </div>
</div>