There has been little research on end-user program development beyond the programming phase. Devising ways to address additional phases may be critical, because research shows that over one-half of the programs written by end users, at least in the widely used spreadsheet paradigm, contain errors. In this paper, we investigate whether providing end users with integrated support for requirement specifications in the form of assertions can help them reason about, recognize, and remove errors in their spreadsheets. Our think-aloud study revealed that end users can indeed use assertions to find and correct errors as they work with their spreadsheets, and also revealed some surprising tendencies and biases about testing.