Building is an integral part of the software development process. However, little is known about the errors occurring in this process. In this paper, we present an empirical study of 26.6 million builds produced during a period of nine months by thousands of developers. We describe the workflow through which those builds are generated, and we analyze failure frequency, error types, and resolution efforts to fix those errors. The results provide insights on how a large organization build process works, and pinpoints errors for which further developer support would be most effective.