Significance Emerging coronaviruses (CoVs; severe acute respiratory syndrome-CoV and Middle East respiratory syndrome-CoV) pose serious health threats globally, with no specific antiviral treatments available. These viruses are able to faithfully synthesize their large genomic RNA. We report, however, that their main RNA polymerase, nsp12, is not accurate. To achieve accuracy, CoVs have acquired nsp14, a bifunctional enzyme able to methylate the viral RNA cap [methyltransferase (MTase)] and excise erroneous mutagenic nucleotides inserted by nsp12. Strikingly, ribavirin can be excised from the viral genome, thus showing no antiviral activity. The crystal structure of nsp14 shows that it is unique, having been replaced by other MTase types during evolution. This unprecedented RNA correction machinery has allowed RNA genome size expansion, but also provided potential nucleoside drug resistance to these deadly pathogens.