When working with single journal articles, I noticed that the number of references in the CrossRef article metadata may in some cases be incomplete as compared to the published article.
For example the published article DOI:10.1103/PhysRevB.56.6100 has 18 references, but the Crossref metadata will only list 16. Missing are references to 2 handbooks which could be listed with a proper ISBN.
Not all reference DOIs in the CrossRef article metadata may be resolved. For example this article DOI:10.1063/1.358185 should reference in CrossRef metadata key->r2 this DOI:10.1103/PhysRevB.56.6100, but is unable. The article DOI is in the CrossRef dataset though.
When conducting automated bibliographic research, solving the inaccuracies may be relevant.