So here's the ECM:

* Node

#+BEGIN_SRC emacs-lisp
  (progn (progn foo
bar))
#+END_SRC

It doesn't matter whether `bar))` is preceded, in the org file, by two tabs
or 16 spaces. In both cases it will be preceded by one tab followed by 7
spaces in the texi file; and by 5 spaces followed by one tab followed by 6
spaces in the info file.

By the way, if I set `tabs-indent-mode` to `nil` in the org file, then tabs
are still used to indent code blocks. Is there a way to enforce the use of
spaces?