M-RET and C-RET turn current line of text into a heading?

[Eric Abrahamsen <eric@ericabrahamsen.net>]

For the past couple of weeks I'm finding that both M-RET and C-RET turn
the line under point into a heading, instead of inserting a new heading
elsewhere. This happens with `org-M-RET-may-split-line' set to anything.

So this:

#+begin_src org
* Chapter One
:PROPERTIES:
:some_prop: t
:END:
In which not [point is here] very much happens. But this is a further test to see
what happens on multiline text.
#+end_src

becomes:

#+begin_src org
* Chapter One
:PROPERTIES:
:some_prop: t
:END:
* In which not very much happens. But this is a further test to see
what happens on multiline text.
#+end_src

This also happens with emacs -Q. Has no one else seen this?

Thanks,
Eric