When you test your product (software or otherwise), you should perform a usability test with people who share some characteristics with your user base. For example, when testing your new website, this characteristic can be "people who use computers". But you may have a particular feature that is very specific. In this case, you should probably test with a more representative group.
For example, your ATM design may include aids for the visually impaired. Your test group, then, should probably include at least one person who is, in fact, blind. This will help to avoid situations where it takes an actual user almost five minutes just to find a headphone jack. And posting a video of it on Youtube.
