I have a frame that refreshes every 10 seconds. The called script does a session_start(), and displays a log-out button when there is a valid session.
After a random number of refreshes, netscape outputs "Document contains no data" instead of the correct content, even though everything is fine with the script. When i reload that specific frame, it works again.
My environment is php 4.0.3pl1 with apache 1.3.12, but a couple of friends of mine ran over that bug too on other, different environments...

Here is the script:
==========================================================
<?$nodebug=1;$need_db=1;include "./lib/config.php";
session_start(); 
?>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">

<html>
<head>
	<title></title>
</head>
<link rel=stylesheet type="text/css" href="./lib/style.css">
<meta http-equiv="refresh" content="15; URL=topbar.php?PHPSESSID=<?if (isset($PHPSESSID)&&$PHPSESSID!="") echo $PHPSESSID;?>">
<body bgcolor="#ffffff" text="#000000">
    <span class="f-8">
        <div align="right">
<? if (isset($PHPSESSID) && $PHPSESSID != "" && $REMOTE_ADDR == $ip) {?>
      logged in<br>
      <a href="logout.php" target="_top">log out</a>
<?}?>
</div>
</span>
</body>
</html>
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config.php sets some variables I use over the whole site.

It happens to other scripts as well, but this frame construction makes it happen quite often (because of the refresh).