Impact analysis based on code dependence can be an integral part of software quality assurance by providing opportunities to identify those parts of the software system that are affected by a change. Because changes usually have far reaching effects in programs, effective and efficient impact analysis is vital, which has different applications including change propagation and regression testing. Static Execute After (SEA) is a relation on program elements (procedures) that is efficiently computable and accurate enough to be a candidate for use in impact analysis in practice. To assess the applicability of SEA in terms of capturing real defects, we present results on integrating it into the build system of Web Kit, a large, open source software system, and on related experiments. We show that a large number of real defects can be captured by impact sets computed by SEA, albeit many of them are large. We demonstrate that this is not an issue in applying it to regression test prioritization, but generally it can be an obstacle in the path to efficient use of impact analysis. We believe that the main reason for large impact sets is the formation of dependence clusters in code. As apparently dependence clusters cannot be easily avoided in the majority of cases, we focus on determining the effects these clusters have on impact analysis.