Thistlethwaite and Campbell (1960) (TC) introduced the Regression Discontinuity Design (RDD) as a strategy for learning about the causal effects of interventions in 1960. Their introduction highlights the most important strengths and weaknesses of the RDD. The main points of the original paper have held up well to more formal scrutiny. However, TC did not address “manipulation of assignment scores” as an important validity threat to the design. The insight that manipulation is a central validity threat is the most important conceptual advance in the methodological literature since its introduction. Although most modern RDD analyses include density tests for assessing manipulation, results are most convincing when diagnostic probes are used to address specific, plausible threats to validity. In this paper, we examine validity threats to two common RD designs used to evaluate the effects of No Child Left Behind and state pre-kindergarten programs.