The pachycephalosaur Stegoceras was confused shortly after its initial description (Lambe 1902) with Troodon, a theropod clade initially known from crowns that resemble ornithischian premaxillary crowns.  This was clarified by Sternberg (1945) for the few members of the clade that were known at that time.  Pachycephalosauridae eventually included only a subset of the larger clade, which came to be called  Pachycephalosauria.

As an aside, Stegoceras validus is the original spelling of the first named genus and species (Lambe 1902).  Sues and Galton (1987) corrected the spelling of the species to validum, so that its gender (neuter) matches that of the genus, in accordance with the ICZN (1999, Art. 34b; Appendix D, Table 2).  This does not change the authorship of the genus and species, which remains Lambe (1902).  Lambe (1918) has often been cited incorrectly for either the genus or species.

Pachycephalosauridae was restricted to domed pachycephalosaurians (Sereno 1986), using the completely known skull of Stegoceras validum as a specifier (Sereno 1998).  The active definition is a first-order revision of that in Sereno (1998) that uses species as specifiers.