SharePoint:WebPartPageUserException This page has encountered a critical error

遇到如下webpart莫名错误，很常见吧。一般用户是直接删掉，知道原因的不算太多。

解决办法(Solution)：

Usually, This error caused by wrong entries under <SafeControl> section in web.config. Remember, Web.config is case sensitive. Instead of "TypeName" if you place "typename" it could lead you to above error!!! Check for Invalid entries in web.config file in order to avoid this error.

Other possibility is: If the site template you are using is not registered as safe in web.config or the feature which you try to install does not register the safe entries may cause this issue.

Solutions:
In my case, Restoring web.config from earlier backups solved this issue. 
If you don't have web.config (You are a bad SharePoint Administrator!) Take every entry out of <SafeControl> section, and try adding them one by one. Stop and find the culprit!

from:http://www.sharepointdiary.com/2012/01/fix-for-this-page-has-encountered.html