Invalid Mark-up Warning - Overlapping or Unclosed Tag

QUESTION:  When the following code is placed into a PHP document created by Dreamweaver an invalid mark-up warning is received and several of the HTML tags are highlighted with yellow.  According to the warning an overlapping or unclosed tag is present.  Unfortunately, I cannot find an overlapping or unclosed tag. 

In an effort to determine the presence or absence of extraneous characters, I cut the code, placed it into a text file, and removed all formatting.  I then pasted it back into the document, saved the document, and still the invalid mark-up warning occurred.

1) What is going on?

2) Should I just ignore it?

With the exception of an alignment problem of the headerInclude and Mainpage divisions everything appears fine.

CODE:

<body class="oneColElsCtr">

     <div id="container">

          <div id="headerInclude">

               <?php include("header.php"); ?>   

          <!-- end #headerInclude --></div>

          <div id="mainContent">

               <h1>The Include( ) Function</h1>   

               <p>This is the content of my Mainpage.  The header to my Mainpage is created with a server-side include using the PHP include( ) function.</p>

           <!-- end #mainContent --></div>

       <!-- end #container --></div>

</body>