1. A method of clustering failing runs in a design verification environment to aid in determining causes of the failing runs, the method comprising: determining a plurality of failing runs of the design verification environment; generating a plurality of partitions of the plurality of failing runs, each respective partition partitions one or more subsets of the plurality of failing runs into one or more non-overlapping clusters of failing runs, wherein each core cluster of the one or more non-overlapping clusters represents a different bug; merging the plurality of partitions of the plurality of failing runs into a hierarchical structure that includes at least one of the clusters; and selecting one or more clusters of failing runs from the merged hierarchical structure.