Final answer:
A false negative or Type I error occurs when an authentication system wrongfully denies access to an authorized user. This can happen due to several reasons, including sensor malfunctions or changes in user data. Enhancing system accuracy helps reduce such errors.
Step-by-step explanation:
When a system denies access to a user who should actually be allowed on the system, the term used to describe this occurrence is a false negative or a Type I error. This happens when the authentication system incorrectly identifies a legitimate user as unauthorized. The example given in the question describes an instance where a user, who was previously able to access the data center by swiping her finger on a biometric sensor, is inexplicably denied entry upon her return, and upon a second attempt, is granted access.
This is indicative of a false negative, which can be caused by a number of factors such as changes in the user's biometric data, temporary malfunctions in the sensor, or even incorrect configuration of the security system. Improving the accuracy of biometric systems can help reduce the incidence of false negatives, thereby enhancing both security and user experience.