\documentclass{article}
\begin{document}

% column 70: ========================================================>
sab ============================================================= sab hello

Steps to reproduce:
\begin{itemize}
\item Add an abbrev named ``sab'', or replace ``sab'' above with an
  existing abbrev of yours.
\item Execute
  (progn
    (setq fill-column 70)
    (add-to-list 'load-path "/path/to/auctex")
    (require 'auctex)
    (LaTeX-mode)
    (abbrev-mode))
\item Place point somewhere on line 5:
  (goto-line 5)
\item Press `M-q' (fill-paragraph)
\end{itemize}

What you will see is that the second ``sab'' will get expanded, and
not the first one.  This is also inconsistent, only LaTeX-mode will do
it.  The built-in latex-mode, tex-mode, text-mode, prog-mode will all
not do it (remember to activate (abbrev-mode) when testing other
modes).

\end{document}