If you're developing in SharePoint, you're probably aware of the useless error messages which are generated if something goes wrong with your code. Well, you can turn on "Friendly Error Messages" by modifying the web.config file of the root SharePoint site. This is NOT recommended for production servers.

1. On your server, find the InetPub directory which hosts your root SharePoint site (usually C:\Inetpub\wwwroot\wss\VirtualDirectories\80)
2. Edit the web.config file and find the following 2 lines:
   1. <SafeMode MaxControls="200" CallStack="false"     Set this to True è     <SafeMode MaxControls="200" CallStack="true"
   2. <customErrors mode="On" />                Set this to Off è     <customErrors mode="Off" />
3. Save the web.config

Then perform an IISReset (Start | Run | cmd | iisreset)