Agrammatism––treated as a medical disorder––is defined as ‘a type of aphasia, usually caused by cerebral disease, characterized by an inability to construct a grammatical or intelligible sentence while retaining the ability to speak single words’. But there should be a different––wider––understanding of agrammatism as the regular incidence of ungrammatical forms in the speech of native speakers arising from imperfect learning.
Such incorrect forms may, of course, enter the canon willy-nilly by being sufficiently well-attested to warrant recognition as alternatives to the standard. Here, for instance, is the case of the past passive form of the verb enamor, used as an adjective (as cited in the Merriam-Webster Unabridged Dictionary Online), where the complement of is the traditional form and with the (formerly) erroneous one:
1: to inflame with love : charm, captivate — usually used in the passive with of <tourists were enamored of the town> and sometimes with with <a beautiful Indian girl with whom he was enamored — Walter Havighurst>
2: to cause (someone) to feel a strong or excessive interest or fascination — usually used in the passive with of or with < … kids who grew up enamored of both Black Sabbath and Black Flag … — Gillian Garr, Rolling Stone, 19 May 1994> <In 1999 Wall Street was enamored with anything dot-com … — Donna Seaman, Booklist, July 2003>
This particular example is relatively benign. But the wholly erroneous use of the Italian borrowing graffiti as a singular instead of graffito(an error compounded by the use of graffitis as a plural [sic!]), heard emanating this morning from the mouth of a published author on the American Public Media radio program “Marketplace Morning Report” (and noticed publicly in an ameliorative tag by the show’s literate host), can only be adjudged agrammatistically wholly beyond the linguistic pale.