[greengo86]

Заменил во всём документе:

;(function($, window, document, undefined){

	// our plugin constructor
	var OnePageNav = function(elem, options){
		this.elem = elem;
		this.$elem = jQuery();
		this.options = options;
		this.metadata = this.$elem.data('plugin-options');
		this.$win = jQuery();
		this.sections = {};
		this.didScroll = false;
		this.$doc = jQuery();
		this.docHeight = this.$doc.height();
	};

	// the plugin prototype
	OnePageNav.prototype = {
		defaults: {
			navItems: 'a',
			currentClass: 'current',
			changeHash: false,
			easing: 'swing',
			filter: '',
			scrollSpeed: 750,
			scrollThreshold: 0.5,
			begin: false,
			end: false,
			scrollChange: false
		},

		init: function() {
			// Introduce defaults that can be extended either
			// globally or using an object literal.
			this.config = jQuery.extend({}, this.defaults, this.options, this.metadata);

			this.$nav = this.$elem.find(this.config.navItems);

			//Filter any links out of the nav
			if(this.config.filter !== '') {
				this.$nav = this.$nav.filter(this.config.filter);
			}

			//Handle clicks on the nav
			this.$nav.on('click.onePageNav', jQuery.proxy(this.handleClick, this));

			//Get the section positions
			this.getPositions();

			//Handle scroll changes
			this.bindInterval();

			//Update the positions on resize too
			this.$win.on('resize.onePageNav', jQuery.proxy(this.getPositions, this));

			return this;
		},

		adjustNav: function(self, $parent) {
			self.$elem.find('.' + self.config.currentClass).removeClass(self.config.currentClass);
			$parent.addClass(self.config.currentClass);
		},

		bindInterval: function() {
			var self = this;
			var docHeight;

			self.$win.on('scroll.onePageNav', function() {
				self.didScroll = true;
			});

			self.t = setInterval(function() {
				docHeight = self.$doc.height();

				//If it was scrolled
				if(self.didScroll) {
					self.didScroll = false;
					self.scrollChange();
				}

				//If the document height changes
				if(docHeight !== self.docHeight) {
					self.docHeight = docHeight;
					self.getPositions();
				}
			}, 250);
		},

		getHash: function($link) {
			return $link.attr('href').split('#')[1];
		},

		getPositions: function() {
			var self = this;
			var linkHref;
			var topPos;
			var $target;

			self.$nav.each(function() {
				linkHref = self.getHash(jQuery());
				$target = jQuery();

				if($target.length) {
					topPos = $target.offset().top;
					self.sections[linkHref] = Math.round(topPos);
				}
			});
		},

		getSection: function(windowPos) {
			var returnValue = null;
			var windowHeight = Math.round(this.$win.height() * this.config.scrollThreshold);

			for(var section in this.sections) {
				if((this.sections[section] - windowHeight) < windowPos) {
					returnValue = section;
				}
			}

			return returnValue;
		},

		handleClick: function(e) {
			var self = this;
			var $link = jQuery();
			var $parent = $link.parent();
			var newLoc = '#' + self.getHash($link);

			if(!$parent.hasClass(self.config.currentClass)) {
				//Start callback
				if(self.config.begin) {
					self.config.begin();
				}

				//Change the highlighted nav item
				self.adjustNav(self, $parent);

				//Removing the auto-adjust on scroll
				self.unbindInterval();

				//Scroll to the correct position
				self.scrollTo(newLoc, function() {
					//Do we need to change the hash?
					if(self.config.changeHash) {
						window.location.hash = newLoc;
					}

					//Add the auto-adjust on scroll back in
					self.bindInterval();

					//End callback
					if(self.config.end) {
						self.config.end();
					}
				});
			}

			e.preventDefault();
		},

		scrollChange: function() {
			var windowTop = this.$win.scrollTop();
			var position = this.getSection(windowTop);
			var $parent;

			//If the position is set
			if(position !== null) {
				$parent = this.$elem.find('a[href$="#' + position + '"]').parent();

				//If it's not already the current section
				if(!$parent.hasClass(this.config.currentClass)) {
					//Change the highlighted nav item
					this.adjustNav(this, $parent);

					//If there is a scrollChange callback
					if(this.config.scrollChange) {
						this.config.scrollChange($parent);
					}
				}
			}
		},

		scrollTo: function(target, callback) {
			var offset = jQuery().offset().top;

			jQuery().animate({
				scrollTop: offset
			}, this.config.scrollSpeed, this.config.easing, callback);
		},

		unbindInterval: function() {
			clearInterval(this.t);
			this.$win.unbind('scroll.onePageNav');
		}
	};

	OnePageNav.defaults = OnePageNav.prototype.defaults;

	jQuery.fn.onePageNav = function(options) {
		return this.each(function() {
			new OnePageNav(this, options).init();
		});
	};

})( jQuery, window , document );

При клике на второй уровень выпадающего списка ни одной ошибки в консоли, но подменю второго уровня всё равно не выезжает =)