Given the experimental nature of information retrieval, progress critically depends on analyzing the errors made by existing retrieval approaches and understanding their limitations. Our research explores various hypothesized reasons for hard topics in TREC-8 ad hoc task, and shows that the bad performance is partially due to the existence of highly distracting subcollections that can dominate the overall performance. Categories and Subject Descriptors H.3.3 [Information Storage and Retrieval]: Error analysis and performance evaluation General Terms Measurement, Performance Keywords Error analysis, hard topics, sub-collection, measures, dominance, distraction