Moving program code that implements cross-cutting concerns into aspects can improve the maintainability of legacy systems. This kind of refactoring, called aspectualization, can also introduce faults into a system. A test driven approach can identify these faults during the refactoring process so that they can be removed. We perform systematic testing as we aspectualize commercial VLSI CAD applications. The process of refactoring these applications revealed the kinds of faults that can arise during aspectualization, and helped us to develop techniques to reduce their occurrences.