This paper investigates the inclusion of “Coptic-only” letters in the spelling of Egyptian personal names and toponyms in otherwise Greek documents. A diachronic analysis of eighty documentary texts (4th–8th c. CE), primarily on papyrus, shows an increase of evidence in the sixth century, in line with recent literature on the evolution of documentary Coptic. As opposed to earlier papyri, which were mainly everyday texts with highly problematic Greek and interference from Egyptian, many later documents were of higher legal value, penned by bilingual scribes who were proficient in Greek, who could incorporate Coptic characters into their Greek writing, proving that script-mixing could be unrelated to poor linguistic competence. The phenomenon seems to have arisen from an unconscious cognitive process of ad hoc phoneme-to-grapheme conversion, which offered different spelling variants, and was triggered by the Egyptian origin of names, the special phonemes that certain Coptic graphemes represented, and the lack of inflection amidst the Greek text.