Glenn Morris <rgm@gnu.org> writes:

> This is annoying...
> AFAICS, this issue appeared in 24.1, but does not go away on loading
> fill.el and make-mode.el from 23.4. But it does go away under edebug.
> Grr.

I have bisected the problem (with git) to this commit:

Author: Stefan Monnier <monnier@iro.umontreal.ca>
Date:   Wed Sep 8 18:21:23 2010 +0200

    Misc cleanups and simplifications.

I have attached the full commit that introduced the bug.

Sincerly
Thomas Christensen