Full Idea
In standard possible worlds semantics the semantic value of Hllary Clinton's utterance of 'I' will be the same as her utterance of 'Hillary Clinton'. But clearly the English word 'I' is not synonymous with the name 'Hillary Clinton'.
Gist of Idea
Possible worlds make 'I' and that person's name synonymous, but they have different meanings
Source
Laura Schroeter (Two-Dimensional Semantics [2010], 1.1.1)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.4
A Reaction
This problem was spotted by Kaplan, and it has been a chief motivator for the creation of two-dimensional semantics, which some people have then extended into a complete semantic theory. No purely extensional semantics can be right.