In Mendelson's book ("Introduction to mathematical logic") he defines truth values for sentences in the propositional calculus using truth tables. However, it seems to me he assumes implicitly that every well-formed sentence (what he calls "statement form") has a unique parsing; i.e. it is impossible for the same statement form to arise in two different ways.
This is of course correct, but it requires a proof. The omission of such proof (or even mentioning it is needed) is somewhat surprising for me as Mendelson's book is otherwise very explicit about everything. Am I missing something?