SharePoint Error "An unexpected error has occurred"

很基础的问题...... 不过当初还是去找了 in here 
The solution is to change a single entry in web.config, by modifying the line…

<SafeMode MaxControls=“200“ CallStack=“false“…

to…

<SafeMode MaxControls=“200“ CallStack=“true“…

You will also need to set custom errors to 'Off' .

<customErrors mode=“Off“/>

You will no longer see the “An unexpected error has occurred” error page and instead you get a lovely ’standard ASP.Net error page’ with the stack trace and everything…development has got that little bit easier!!