The original aim of the present study was to add to the discussion on the phonological status of the Tibetan grapheme a-chun written in Tibetan script in non-Tibetan texts. As a result, the article argues that Sinologist Weldon S. Coblin was substantially right in stating that "neither 'nasalization' nor anything else can explain all the varied foreign elements represented in our data by ' a-chung plus following consonants" (Coblin 2002: 181). Thus, according to Coblin's statement, at least in some contexts, the grapheme achun seems to occur quite randomly. Some scholars have argued that this is a consequence of careless and loose employment of the Tibetan alphabet, a widespread habit among many copyists from Dunhuang. In this article, this historical perspective is challenged by applying a new approach to the allographic context exhibited in Ms. Paris, BnF, Pelliot tibetain 1292, an Old Turkic text written in Tibetan script. Far from being a poorly transliterated text, it will be shown to be the complex (and obviously not entirely consistent) product of a "hyperphonetic" transcription process: a sort of phonetic pronunciation guide for non-Turkic Buddhist monks.