The original define_main_page.php contains:
and is used to build part of the main page of the site, so if the contents are corrupt or invalid, it will effect the contents of the main page.
<p><strong>Main Page Sample Text ...</strong></p>
<p>This section of text is from the Define Pages Editor located under Tools in the Admin.</p>
<p>To remove this section of the text, delete it from the Define Pages Editor.</p>
If you look at the Holzheimer's site in my sig, the portion of the main page that comes from define_main_page.php begins with:
<center>* * * * * * * * * * * * * * * *</center>
Although our selection of merchandise changes frequently, we
make every effort to fill your order completely.