Shell scripts: undefined -a, -o switch behavior
<p>According to the <a href="https://github.com/koalaman/shellcheck/wiki/SC2166" rel="nofollow">shellcheck wiki</a>:</p>

<blockquote>
  <p><code>-a</code> and <code>-o</code> to mean <code>AND</code> and <code>OR</code> in a [ .. ] test expression is not well defined. They are obsolescent extensions in POSIX and their behavior is almost always undefined.</p>
</blockquote>

<p>Any idea what wrong can happen? </p>