The Greek texts from Egypt show extensive nonstandard vowel production, which could cause inadvertent confusion in e.g. Greek mood or case endings. This has previously been seen as evidence of a bad command of Greek, either because of internal phonological change or due to imperfect knowledge of Greek. On closer look numerous similarities to the nonstandard vowel production in Greek texts can also be found in native (Coptic) Egyptian texts. Greek loanwords in Coptic are treated according to Coptic phonological rules and show nonstandard vowel usage of the same nature that is present in Greek in some sociolects. The nonstandard spellings present evidence of underdifferentiation of Greek phonemes as well as transfer elements of the Egyptian prosodic system. The vowel usage is examined within the framework of L2WS (second language writing systems) studies, and evidence for the coarticulatory effect of the consonants on the vowels’ quality is drawn from the field of articulatory phonetics.