<p>Here are the main reasons why commented code is a code smell :</p>
<ul>
    <li>It always raises more questions than it gives answers</li>
    <li>Everybody will forget very quickly how relevant the commented code is</li>
    <li>This is distraction when going down the code as it stops the flow of eyes</li>
    <li>It is a bad SCM engine : Subversion, CVS and Git are really more trustworthy !</li>
    <li>The simple fact of understanding why code was commented out in the first place can take a lot of time</li>
</ul>