Someone cannot tell the pronunciation of "th" and "s" apart. I am sure there's a single word that can express this in the context. But I can't remember it now. Anyone can help?
There isn't one word that refers to that particular linguistic problem, but cf. the word shibboleth:
A word or custom whose variations in pronunciation or style can be used to differentiate members of ingroups from those of outgroups. Within the mindset of the ingroup, a connotation or value judgment of correct/incorrect or superior/inferior can be ascribed to the two variants.