Although there are many papers examining ambiguity in Information Retrieval, this paper shows that there is a whole class of ambiguous word that past research has barely explored. It is shown that the class is more ambiguous than other word types and is commonly used in queries. The lack of test collections containing ambiguous queries is highlighted and a method for creating collections from existing resources is described. Tests using the new collection show the impact of query ambiguity on an IR system: it is shown that conventional systems are incapable of dealing effectively with such queries and that current assumptions about how to improve search effectiveness do not hold when searching on this common query type. Categories and Subject Descriptors H.3.3 [Information Search and Retrieval]: Systems and Software --- performance evaluation. General Terms Measurement, Experimentation. Keywords Ambiguous queries, test collections, diversity.