This article, embedded in the field of Philology -with an historical approach and a linguistic viewpoint-and in the history of the Spanish language, discusses the acceptability of the examples presented as copyist errors in two of the most renowned manuals in Spanish textual criticism, namely, Manual de critica textual (1983, 2001) by Alberto Blecua, and La edicion de textos (2011) by Miguel Angel Perez Priego (for Perez Priego, see Arellano's observations in his review, 1997). The analysis of the cases and their documentation from different sources -critical editions and metalinguistic works-enables the conclusion that several of the so-called errors in these manuals are in fact legitimate, historically-documented linguistic variations, although some of them being scarcely testified.