Over-Requirement occurs in software development projects when a software product is specified beyond the actual needs. This study shows empirically that Over-Requirement happens partially due to the Planning Fallacy, i.e., the tendency of people to underestimate the time needed to complete a task. Underestimating the time needed to develop a software feature during project planning, we argue, may lead to including within the project scope more required and unrequired features than can be completed by the project deadline. To investigate this argument, we conducted an experiment in which participants were asked to estimate the time it would take to develop various software features in a software development project and then, given the project's duration, to recommend which of the features to include within scope. The results confirmed that the Planning Fallacy occurs in the context of software development and influences the Over-Requirement phenomenon.