Full Idea
The causal theory of reference, in a full-blown form, makes it impossible to distinguish between knowing the use of a proper name and simply having heard the name and recognising it as a name.
Gist of Idea
The causal theory of reference can't distinguish just hearing a name from knowing its use
Source
Michael Dummett (Frege's Distinction of Sense and Reference [1975], p.254)
Book Reference
'Meaning and Reference', ed/tr. Moore,A.W. [OUP 1993], p.254
A Reaction
None of these things are all-or-nothing. I have an inkling of how to use it once I realise it is a name. Of course you could be causally connected to a name and not even realise that it was a name, so something more is needed.