GNU Emacs 25.0.50.1 (i686-pc-linux-gnu, GTK+ Version 3.14.4) of 2014-10-26
emacs -Q, then (setq org-src-fontify-natively t) and try visiting an org file with source code blocks, the message "org-mode fontification error" will appear and no source code will be highlighted. The issue seems to go away if I install Org-mode from ELPA afterwards.