In following example, dropdown menu works. It will fail when we rename the bug2 to bug(same as href in <a>), check it in jsbin
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css">
<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap-theme.min.css">
</head>
<body>
<div class="navbar navbar-default" role="navigation">
<div class="container">
<div class="navbar-header">
<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target=".navbar-collapse">
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="#">Bootstrap theme</a>
</div>
<div class="navbar-collapse collapse">
<ul class="nav navbar-nav">
<li class="dropdown">
<a href="#bug" class="dropdown-toggle" data-toggle="dropdown">
Dropdown <span class="caret"></span></a>
<ul class="dropdown-menu" role="menu">
<li><a href="#">Action</a></li>
<li><a href="#">Another action</a></li>
</ul>
</li>
</ul>
</div>
</div>
</div>
<div id="bug2">main</div>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script src="//maxcdn.bootstrapcdn.com/bootstrap/3.2.0/js/bootstrap.min.js"></script>
</body>
</html>
SOLVED
For keep same style, the corrected version is replace this line
<a href="#bug" class="dropdown-toggle" data-toggle="dropdown">
with
<a href="#" class="dropdown-toggle" data-toggle="dropdown">