When a text field has focus and Enter is pressed the form will automatically submitted. This happened even if there is no submit buttons and even with normal button. But if you make a normal text field and set its style (CSS) display to none, the form will not be submitted.

<body>
<form>
<input name=”txt1″ />
<input type=”button” value=”Submit” />
<input type=”text” style=”display:none” />
</form>
</body>

The form will still be submitted if you use a hidden text field.