Consider the following story;
As a web master, I want a captcha so that we reduce spam
This looks like a reasonable story. However, I think there is a problem with this story - the web master role is incidental.
What I mean is the web master role is incidental to the implementation technology. If we changed the technology we implemented our solution in the role of web master would be replaced.
However, the requirement to reduce spam by some sort of manual input (captcha) would remain.
The real customer in this example could be the marketing department or customer service who don't want to waste time on customers that aren't real.