Error message not shown even after setting CustomErrors in web.config

We know that if we want to see the details of error message, we can change the
<customErrors mode="On" />  to Off and we can see the exact error message.

But wait - ever after changing this to Off in web.config file - it still shows this page below:

Well the solution is to edit the web.config file in this location with <customErrors mode="Off" />  i.e.

C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS

After changing the web.config, refresh the page and you will see the specific error message.