This document compares several bibliographic data sources and finds substantial discrepancies between them. It analyzes the coverage of publications and citations in Web of Science, Scopus, Dimensions, and Crossref. Dimensions and Scopus have the most complete coverage of publications, while Crossref is incomplete due to closed or missing citations. Pairwise comparisons reveal millions of citations that are unique to each source. The causes of discrepancies include reference inaccuracies, versioning issues, and different matching algorithms. Examples demonstrate problems caused by group authors and supplements in Web of Science.