Reliability of citation searches is a cornerstone of bibliometric research. The authors compare simultaneous search returns at two sites to demonstrate discrepancies that can occur as a result of differences in institutional subscriptions to the Web of Science and Web of Knowledge. Such discrepancies may have significant implications for the reliability of bibliometric research in general, but also for the calculation of individual and group indices used for promotion and funding decisions. The authors caution care when describing the methods used in bibliometric analysis and when evaluating researchers from different institutions. In both situations a description of the specific databases used would enable greater reliability.