When I was going through some of the photocopied material I have from the Rudolf Carnap archive, I found the following interesting (to me) remark. Carnap is discussing with Nelson Goodman a draft of Goodman's dissertation (A Study of Qualities, which much later became The Structure of Appearance). Goodman points out various supposed technical defects with Carnap's method of quasi-analysis presented in the Aufbau, and Carnap says roughly the following:
The real problem with my Aufbau is not the various counter-examples that can be constructed against my particular version of quasi-analysis (which I already knew about), but rather the assumption of extensionality.
(I haven't quoted, because I don't think it's allowed.) I like this because (a) it's a really smart philosopher saying 'My book rests on a mistake,' and (b) it fits with my pet theory that the real break between Carnap and Quine stems in large part from Carnap moving away from the extensional languages he espoused in Logical Syntax, while Quine held fast to the extensional standard throughout his life (see the posthumous "Confessions of a Confirmed Extensionalist").