This paper deals with syntactic blendings, or ‘syntactic grafts’, that is to say, speech errors whereby parts of competing formulations are interwined in speech (for example: la manifestation s’annonce pour être exceptionnelle). In fact, blending is a general process : it also operates both at the word level (portemanteau words), and at the collocational level. In order to distinguish between syntactic blending from analogical extension, we study the role played by phraseological units in the wording of syntactically deviant sentences : it appears that conventional units and formulaic language provide the necessary interface between productivity within the linguistic system (analogy/productivity) and creativity in speech.