move-beginning-of-line moves to the very beginning instead of where it should

[dan]

When i type control-A (move-beginning-of-line) in a shell or in the minibuffer,
it normally moves the insertion point right to where it would make sense to type
(i.e., right after the prompt).

However, sometimes it seems to lose its way, and when it does, it quits working
everywhere (shell, minibuffer), and goes all the way to the beginning of the
line, rather than right after the prompt.

This happens on mac os x, with emacs 24.3.1 and with 22.1.1 (the one
bundled with the mac).  It takes from a day to a few weeks or so to show up.

I've never had that experience with ubuntu, despite the fact that my
usage is similar (using emacs continuously, and leaving it up for
months).

I've opened dribble files so that i can go back and try to see if i typed
something funny to cause it.  But what i type looks innocuous (all key
combos i can recognize, at least the most recent few hundred).

So i'm wondering what i'm doing that changes the meaning of
move-beginning-of-line, and also how i can restore it to go
to the position right after the prompt instead of the very
beginning of the line.  This happens on both 22.1.1 and 24.3.1,
but is worse on 22.1.1 (where it makes escape-p malfunction in
shell mode).

I tried googling for this several times, but never found it.
However i could not possibly be the only person who experiences it.)

Thanks in advance for any pointers.

dan