Randall, J. E., & McCarthy, L. J. (1989). Solea stanalandi, a new sole from the Persian Gulf. Japanese Journal of Ichthyology, 36(2), 196–199. doi:10.1007/BF02914322
is published by Springer with the DOI http://dx.doi.org/10.1007/BF02914322, and this DOI is registered with CrossRef. J-Stage publish the same article, with the DOI (http://dx.doi.org/10.11369/jji1950.36.196). This DOI is not registered with CrossRef. I haven't been able to find an easy way to discover the DOI registration agency for a DOI (surely there should be a simple service that tells me this?).
This illustrates a problem with the success of DOIs and the existence of multiple registration agencies. When there was essentially a single agency for publications (CrossRef) it was relatively easy to ensure that DOIs for publications were unique. Now that there are multiple DOI registration agencies it is possible for conflicts to arise. We might expect this to be rare, after all, surely there's only one publisher for an article? However, the publishing landscape is more complicated that that, with articles being served by multiple publishers, and archiving projects like JSTOR and BHL having content that overlaps with that of existing publishers. Messy (sigh).