> That aside, I can't see how the two assertions suggested in #18913 could
> trigger the message I saw.  I'll maybe have a look at the actual patch
> sometime when I'm less tired.

There is a third assertion added in [1: 62ee8b1a59] which is the one you
hit.

[1: 62ee8b1a59]: 2014-10-31 16:57:59 -0400
  * lisp/simple.el (newline): Add assertions to try and help catch bug#18913.
  http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=62ee8b1a59d5343197cb59d9f0a52e07a23af1d8