* Summary

With visual-line-mode and org-indent-mode activated, C-a and C-e
misbehaves on indented text blocks.  C-a and C-e sometimes start jumping
over lines (ie, go backwards or forward one extra line) and sometimes
even reverses direction (C-e goes backward). I can reproduce this from
emacs -Q with builtin Org 8.2.10. The issue can also be reproduced
without enabling org-indent-mode, although it is much less severe
without it.

* Steps to reproduce

From emacs -Q:

 1. Open the attached document, or create a new org-mode file with an
    heading and a (preferably long) paragraph or text under it.
 2. Enable visual-line-mode and org-indent-mode
 3. Reduce Emacs frame width.
 4. With text-scale-mode or (set-frame-parameter), seriously increase
    the font size (if it can't display more than two or three words per
    line, that's perfect).
 5. Go to the last character of the huge paragraph.
 6. Start playing with C-a and C-e.  The cursor will wander erratically
    within the paragraph, often in the wrong direction.

Notice: setting the frame width, the font size and going to the end of
the document is not strictly required, as the bug will appear sooner or
later, but in my tests it made it appear much faster, in a matter of
seconds.]

I'm using Emacs 25.2rc2 with org 9.0.5 from ELPA, but have been
experiencing this since Emacs 24.5 and the built-in Org (8.x I believe).
I can reproduce under the same conditions with the built-in org of Emacs
25.2rc2 as well.

Thanks a lot for your help,
Thibault