Error while modifying Content Query Web Part

Issue:

User tried to modify Content Query Web Part and got following error:

An unexpected error has occurred. Web Parts Maintenance Page: If you have permission, you can use this page to temporarily close Web Parts or remove personal settings. For more information, contact your site administrator

Resolution:

• Open the page in SPD where you put the web part
• Right click on web part and click web part properties
• 
  It will give you following error

•  This is the first step where you can narrow down the issue

• Open Style Library > XSL Style Sheets
• There is a file called itemstyle.xsl
• Open that file in designer
• Press ctrl + F and select “Replace all” option type nbsp
• Replace it with &#160
• Save it and try again to modify the web part
• Let me know if you have any queries on above steps

Recommendation:

Before making any change to any file from SPD please take backup of that particular file, this will be a real SAVIER in such situations

The best link I found for customizing Content Query Web Part – http://www.heathersolomon.com/blog/articles/customitemstyle.aspx