I couldn't reproduce this with emacs -Q, but this is what happends. I insert these forms into a scratch-buffer

;; QUIT gets inserted

(condition-case error

(sleep-for 5)

(quit (goto-char (point-max))

(insert "QUIT")))

;; QUIT doesn't get inserted. Quit isn't displayed in the mini-buffer

(condition-case error

(let ((inhibit-quit t))

(sleep-for 5))

(quit (goto-char (point-max))

(insert "QUIT")))

GNU Emacs 24.3.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.6.4) of 2013-11-02 on ubuntu