CustomErrors mode

During SharePoint development you can get this kind of screen:

There are two places you should look for web.config file and correct the setting to
<customErrors mode="Off" />
to get meaningful details of your error displayed.

First one is obvious: your site's web config. Open the IIS Manager and explore your site in order to edit web.config file:

Second one had made me think for a second and then I decided to create this post. As I was working on application page which was deployed down in _layouts folder, I found there another web config file to be corrected:

Update for SharePoint 2013:

Expand _layouts folder and find the 15 subfolder: there you have to correct anothet web.config file!