I'm starting to figure out tangle by wrapping chunks of my emacs init.el in #+begin_src/#end_src and then hitting C-c C-v t. It has been working fine, but one block simply refused to participate. I eventually tracked the problem down to a stray NULL character that had found its way into some of the lisp comments in that particular chunk of my init.el. It had the effect of completely disabling tangling of the entire block. Blocks before and after that one, however, all tangle nicely.
The attached .org file describes a simple test to demonstrate the problem. I've also attached a .zip version, in case the NULL character in the test doesn't survive the gmailing process. (The null is In BLOCK 2, two characters after the '3' in ';; line3' If it's there, you should see the usual ^@ (as a single character) placeholder.
test-effect-of-null.org
test-effect-of-null.org.zip