C-h f strange behavior with 24.1.50 snapshot

[Leo Alekseyev]

I'm running a 2012-07-01 snapshot from Damien Cassou's Ubuntu PPA and
am seeing C-h f breaking /on one of my machines/ with
: help-fns--autoloaded-p: Wrong type argument: arrayp, nil

The function names which cause this breakage seem to be dependent in
some non-trivial fashion on what I have loaded in my .emacs.  For
instance, with blank .emacs, e.g. C-h f dired works, but if I load one
of (emacs-nav [[http://code.google.com/p/emacs-nav/]], dired-extension
[[http://www.emacswiki.org/emacs/dired-extension.el]]) it throws the
error.  I haven't been able to figure out what in these packages
causes the error.

If I do emacs -Q, I can reproduce this error by doing
: C-h f help-fns--autoloaded-p

What makes this bizarre is that between my two machines, both running
Ubuntu 12.04 64-bit, both with freshly installed emacs-snapshot from
the PPA, only _one_ exhibits the breakage.  How would I try to isolate
the problem further?

In general, how does one go about getting more information about such
errors?  Can one enable a more detailed traceback, like one provided
by --debug-init, but something that would get triggered by errors such
as the one above?