  Genome-scale orthology assignments are usually based on reciprocal best
matches. In the absence of horizontal gene transfer (HGT), every pair of
orthologs forms a reciprocal best match. Incorrect orthology assignments
therefore are always false positives in the reciprocal best match graph. We
consider duplication/loss scenarios and characterize unambiguous false-positive
(u-fp) orthology assignments, that is, edges in the best match graphs (BMGs)
that cannot correspond to orthologs for any gene tree that explains the BMG.
Moreover, we provide a polynomial-time algorithm to identify all u-fp orthology
assignments in a BMG. Simulations show that at least $75\%$ of all incorrect
orthology assignments can be detected in this manner. All results rely only on
the structure of the BMGs and not on any a priori knowledge about underlying
gene or species trees.
