The call to $.ajax should be AJS.$.ajax in the file tinyMce-autocomplete-settings-mentions.js of the Confluence Mentions plugin.

            if (url) {
                xhr = $.ajax({
                    type: "GET",
                    url: url,
                    data: getParams(autoCompleteControl, val),
                    dataType: "json",
                    global: false,
                    timeout: 10000
                });

This may lead to a JS error when using mentions. The errors produced will look like:

• Firefox: TypeError: $ is undefined
• Chrome: Uncaught TypeError: Cannot read property 'ajax' of undefined
• IE10: Unable to get property 'ajax' of undefined or null reference

Workaround

1. Go to the "Custom HTML" section of the Confluence Administration Console.
2. Add the following text to the "At end of the HEAD" section:

   <script type="text/javascript">
   var $ = jQuery;
   </script>