Bootstrap - dropdown menu not working?

Check if you're referencing jquery.js BEFORE bootstrap.js and bootstrap.js is loaded only once. That fixed the same error for me:

<script type="text/javascript" src="js/jquery-1.8.0.js"></script>
<script type="text/javascript" src="js/bootstrap.min.js"></script>

This is a Rails specific answer, but I had this same problem with Bootstrap dropdown menus in my Rails app. What fixed it was adding this to app/assets/javascripts/application.js:

//= require bootstrap

Hope that helps someone.