if some reason two versions of jQuery are loaded (which is not recommended), calling $.noConflict(true) from the second version will return the globally scoped jQuery variables to those of the first version.
Some times it could be issue with older version (or not stable version) of JQuery files
Solution use $.noConflict();
<script src="other_lib.js"></script>
<script src="jquery.js"></script>
<script>
$.noConflict();
jQuery( document ).ready(function( $ ) {
$("#opener").click(function() {
$("#dialog1").dialog('open');
});
});
// Code that uses other library's $ can follow here.
</script>