http://seazbounty.castledragmire.com/bugs/formbug.php reproduces the problem.
To produce the problem, type in anything to the textbox and press enter.  You will notice nothing happens, now type something in and click the submit button.  It now works.  I have reproduced this bug on windows and linux platforms, so it is not a browser problem.

Here's that scripts source code:
<?php
  //formbug.php
  
  print $_SERVER["SERVER_SIGNATURE"]."\n";
  
  if(!isset($_POST["submit"]))
  {
    print "type in anything to the text box and press enter";
    print "<form name=login action=".$_SERVER["PHP_SELF"]." method=post>\n";
    print "<input type=text name=blah>\n";
    print "<input type=submit name=submit value=submit>\n";
    print "</form>\n";
  }
  else
  {
    print "Pressing enter didn't work did it!<br>";
    print "Bug is solved when having more than one field in a form, other than the submit button.";
  }
?>