CKEditor.

The error came from some script in Application.js

public/javascripts/ckeditor/_source/plugins/newpage/plugin.js

@@ -0,0 +1,54 @@
+/*
+Copyright (c) 2003-2011, CKSource - Frederico Knabben. All rights reserved.
+For licensing, see LICENSE.html or http://ckeditor.com/license
+*/
+
+/**
+ * @file Horizontal Page Break
+ */
+
+// Register a plugin named "newpage".
+CKEDITOR.plugins.add( 'newpage',
+{
+	init : function( editor )
+	{
+		editor.addCommand( 'newpage',
+			{
+				modes : { wysiwyg:1, source:1 },
+
+				exec : function( editor )
+				{
+					var command = this;
+					editor.setData( editor.config.newpage_html || '', function()
+					{
+						// Save the undo snapshot after all document changes are affected. (#4889)
+						setTimeout( function ()
+						{
+							editor.fire( 'afterCommandExec',
+							{
+								name: command.name,
+								command: command
+							} );
+							editor.selectionChange();
+
+						}, 200 );
+					} );
+					editor.focus();
+				},
+				async : true
+			});
+
+		editor.ui.addButton( 'NewPage',
+			{
+				label : editor.lang.newPage,
+				command : 'newpage'
+			});
+	}
+});
+/**
+ * The HTML to load in the editor when the "new page" command is executed.
+ * @type String
+ * @default ''
+ * @example
+ * config.newpage_html = '<p>Type your text here.</p>';
+ */