Why does the compiler keeps telling me (forever) to rerun because labels have changed?

Question

I have a LaTeX paper (multifile setup, using \input{} statements) that , since a few days ago, keeps telling me at each compile (both with pdflatex or latex):

LaTeX Warning: Label(s) may have changed. Rerun to get cross-references right

Even recompiling, say, ten times does not fix it. All references in the paper seems fine, though (can't be 100% sure, would need to check each and every \ref in the source with the PDF output).

It's probably because I misplaced a label or reference in some way. But how to find it, and fix the issue? What sort of misplacements result in this repeated message?

Edit: I've tracked it down to an issue with the acronym package in combination with using \ac{} in the abstract definition of an elsarticle (Elsevier's latex class, I am using the latest version, i.e. 1.20b from 2009-09-17) document. Below is a minimal copy/paste example where the issue appears:

\documentclass[final,5p, times,twocolumn]{elsarticle}
%\documentclass{article} --> no problem when using this one (and removing frontmatter)
\usepackage[nolist]{acronym}

\begin{document}
    \begin{frontmatter}
        \begin{abstract}
                This is just a sentence with an acronym in it: \ac{ICT}.
        \end{abstract}
    \end{frontmatter}

    \acrodef{ICT}{Information and Communication Technologies}
\end{document} 

It is not clear to me if this is because of a bug in the acronym package, or elsarticle.

Answer 1

This is actually a bad interaction with the \twocolumn command, and so is a more general problem that just for the elsarticle class. Here is a minimal example triggering the error:

\documentclass[twocolumn]{article}
\usepackage[nolist]{acronym}

\begin{document}
\twocolumn[Heading with acronym \ac{ICT}]{
Some text.}

\acrodef{ICT}{Information and Communication Technologies}
\end{document} 

As @Jouhn Collins notices the .aux file now has \reset@new@label placed too late:

\relax 
\undonewlabel{acro:ICT}
\newlabel{acro:ICT}{{}{1}}
\acronymused{ICT}
\reset@newl@bel
\newacro{ICT}[ICT]{Information and Communication Technologies}

A work around is to add a macro that writes \reset@new@label out to the .aux file. (I am very reluctant to meddle with the definition of \twocolumn. Trying to this at \begin{document} is too late.) Here is such a fix, first in the minimal example:

\documentclass[twocolumn]{article}
\usepackage[nolist]{acronym}

\makeatletter
\newcommand{\extraclearlabels}{\protected@write\@auxout{}{%
  \string\reset@newl@bel
}}
\makeatother

\begin{document}
\twocolumn[\extraclearlabels Heading with acronym \ac{ICT}]{
Some text.}

\acrodef{ICT}{Information and Communication Technologies}
\end{document}

and now in the original example:

\documentclass[final,5p,times,twocolumn]{elsarticle}
\usepackage[nolist]{acronym}
\makeatletter
\newcommand{\extraclearlabels}{\protected@write\@auxout{}{%
  \string\reset@newl@bel
}}
\makeatother

\begin{document}

\begin{frontmatter}
  \begin{abstract}
    \extraclearlabels
    This is just a sentence with an acronym in it: \ac{ICT}.
  \end{abstract}
\end{frontmatter}

In text acronym \ac{ICT}.

\acrodef{ICT}{Information and Communication Technologies}
\end{document}