While this principle is widely known and preached, it is outstanding how often it is overlooked.
And it is more frustrating when it happens on websites that target non-technical users.
Take this example from securecode (an initiative by MasterCard to increase credit card payments over the Internet): during the sign up process, if you enter a wrong credit card number, this is the error message you get
As you may notice, you get more information from the page URL (which you don't see, using most browsers) than from the error message, which is generic and (most of all) false.
The system was, in fact, available. I entered the correct card number and was able to proceed with the registration.
It is because of episodes like this that most people still see computers as mystical and mysterious devices.