Contract-based modular checkers have the potential to perform scalable checking of user-defined properties. However, such tools have seldom been deployed on large software applications of industrial relevance. We present a case study of applying a modular checker HAVOC to check properties about the synchronization protocol of a core Microsoft Windows component with more than 300,000 lines of code and 1500 procedures. The effort found 45 serious bugs in the component with modest annotation effort and low false alarms; most of these bugs have since been fixed by the developers of the module. We describe our experience in using a modular checker to create various property checkers for finding errors in a well-tested application of this scale, and our design decisions to find them with low false alarms, modest annotation burden and high coverage.