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
- 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  
- 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
