Is it a bug?

Let's play Demo on http://www.fckeditor.net/demo,
After you see "This is some sample text. You are using FCKeditor.", click "source" button. you will see "<p>This is some <strong>sample text</strong>. You are using <a href="http://www.fckeditor.net/">FCKeditor</a>.</p>", then click "source" button again, you will see "This is some sample text. You are using FCKeditor." again. Now move the cursor to the end of this sentence and press "delele" key, a lot "bugs" croweds into the editor like this:

This is some sample text. You are using FCKeditor.<!-- function __RP_Callback_Helper(str, strCallbackEvent, splitSize, func){var event = null;if (strCallbackEvent){event = document.createEvent('Events');event.initEvent(strCallbackEvent, true, true);}if (str && str.length > 0){var splitList = str.split('|');var strCompare = str;if (splitList.length == splitSize)strCompare = splitList[splitSize-1];var pluginList = document.plugins;for (var count = 0; count < pluginList.length; count++){var sSrc = '';if (pluginList[count] && pluginList[count].src)sSrc = pluginList[count].src;if (strCompare.length >= sSrc.length){if (strCompare.indexOf(sSrc) != -1){func(str, count, pluginList, splitList);break;}}}}if (strCallbackEvent)document.body.dispatchEvent(event);}function __RP_Coord_Callback(str){var func = function(str, index, pluginList, splitList){pluginList[index].__RP_Coord_Callback = str;pluginList[index].__RP_Coord_Callback_Left = splitList[0];pluginList[index].__RP_Coord_Callback_Top = splitList[1];pluginList[index].__RP_Coord_Callback_Right = splitList[2];pluginList[index].__RP_Coord_Callback_Bottom = splitList[3];};__RP_Callback_Helper(str, 'rp-js-coord-callback', 5, func);}function __RP_Url_Callback(str){var func = function(str, index, pluginList, splitList){pluginList[index].__RP_Url_Callback = str;pluginList[index].__RP_Url_Callback_Vid = splitList[0];pluginList[index].__RP_Url_Callback_Parent = splitList[1];};__RP_Callback_Helper(str, 'rp-js-url-callback', 3, func);}function __RP_TotalBytes_Callback(str){var func = function(str, index, pluginList, splitList){pluginList[index].__RP_TotalBytes_Callback = str;pluginList[index].__RP_TotalBytes_Callback_Bytes = splitList[0];};__RP_Callback_Helper(str, null, 2, func);}function __RP_Connection_Callback(str){var func = function(str, index, pluginList, splitList){pluginList[index].__RP_Connection_Callback = str;pluginList[index].__RP_Connection_Callback_Url = splitList[0];};__RP_Callback_Helper(str, null, 2, func);} //-->


In my project I don't even need to click source twice to get these bugs, I just need to press "delete" directly at the end of my content.