Hello!

The most easy way to reproduce the described behaviour is to launch  
Emacs.app with -Q and then execute/evaluate in its *scratch* buffer a  
bit of Elisp code:

	<extend load-path that the AUCTeX files are found>
	  (load "auctex.el" nil t t)
	  (load "preview-latex.el" nil t t)
	 (setq TeX-PDF-mode t)

With the last line Emacs.app always fails to produce previews. The  
setting can also be made active for a particular buffer by executing  
the TeX-PDF-mode function. The interesting thing is that, when TeX- 
PDF-mode is nil in the buffer you can create a preview, when you set  
it to t you can't, when you set it to nil again, yes, you can again!

--
Greetings

   Pete

When in doubt, use brute force.
				– Ken Thompson