Generally, need-based financial aid improves students' academic outcomes. However, the largest source of need-based grant aid in the United States, the Federal Pell Grant Program (Pell), has a mixed evaluation record. We assess the minimum Pell Grant in a regression discontinuity framework, using Kentucky administrative data. We focus on whether and how year-to-year changes in aid eligibility and interactions with other sources of aid attenuate Pell's estimated effects on post-secondary outcomes. This evaluation complements past work by assessing explanations for the null or muted impacts found in our analysis and other Pell evaluations. We also discuss the limitations of using regression discontinuity methods to evaluate Pell--or other interventions with dynamic eligibility criteria--with respect to generalizability and construct validity.