One of the most common mistakes in deploying an ASP.NET website into a production environment is to leave the Trace and Debug or custom errors attribute enabled. This mistake often lead to lowering the overall performance of your ASP.NET application.
If you want to improve the performance of your application, practice deploying the application and set Trace and Debug attributes to false. And, to improve security, leave Custom Errors attribute to enabled.
If you are a web administrator, and you have several websites, there’s a simple tip to override the deployment mistakes.
Read the rest of the post here: http://www.devcurry.com/2010/11/using-deployment-retailtrue-attribute.html