  We report evidence of an undocumented method to manipulate citation counts
involving 'sneaked' references. Sneaked references are registered as metadata
for scientific articles in which they do not appear. This manipulation exploits
trusted relationships between various actors: publishers, the Crossref metadata
registration agency, digital libraries, and bibliometric platforms. By
collecting metadata from various sources, we show that extra undue references
are actually sneaked in at Digital Object Identifier (DOI) registration time,
resulting in artificially inflated citation counts. As a case study, focusing
on three journals from a given publisher, we identified at least 9% sneaked
references (5,978/65,836) mainly benefiting two authors. Despite not existing
in the articles, these sneaked references exist in metadata registries and
inappropriately propagate to bibliometric dashboards. Furthermore, we
discovered 'lost' references: the studied bibliometric platform failed to index
at least 56% (36,939/65,836) of the references listed in the HTML version of
the publications. The extent of the sneaked and lost references in the global
literature remains unknown and requires further investigations. Bibliometric
platforms producing citation counts should identify, quantify, and correct
these flaws to provide accurate data to their patrons and prevent further
citation gaming.
