What if the application has functionality that wasn’t in the requirements?
It may take serious effort to determine if an application has significant unexpected or hidden functionality, and it could indicate deeper problems in the software development process. If the functionality isn’t necessary to the purpose of the application, it should be removed, as it may have unknown impacts or dependencies that were not taken into account by the designer or the customer. (If the functionality is minor and low risk then no action may be necessary.) If not removed, information will be needed to determine risks and to determine any added testing needs or regression testing needs. Management should be made aware of any significant added risks as a result of the unexpected functionality.
This problem is a standard aspect of projects that include COTS (Commercial Off-The-Shelf) software or modified COTS software. The COTS part of the project will typically have a large amount of functionality that is not included in project requirements, or may be simply undetermined. Depending on the situation, it may be appropriate to perform in-depth analysis of the COTS software and work closely with the end user to determine which pre-existing COTS functionality is important and which functionality may interact with or be affected by the non-COTS aspects of the project. A significant regression testing effort may be needed (again, depending on the situation), and automated regression testing may be useful.