In this paper, we report on our experiences of using lightweight formal methods for the partial validation of natural language (NL) requirements documents. We describe a case study based on part of NASA's specification of the Node Control Software of the International Space Station, and apply to it our method of checking properties on models obtained by shallow parsing of natural language requirements. These experiences support our position that it is feasible and useful to perform automated analysis of requirements expressed in natural language. Indeed we identified a number of errors in our case study that were also independently discovered and corrected by NASA's IV&V Facility in a subsequent version of the same doument. The paper describes the techniques we used, the errors we found, and reflects on the lessons learned. Keywords: Natural language requirements, lightweight formal methods, requirements validation.