I wanted to change the form for posting the page comments. So I have copy&paste this form from a page to the template and the silverstripe was complainig about bad html. E.g. your form generates:
<input class="action" id="PageCommentInterface_Form_PageCommentsPostCommentForm_action_postcomment" type="submit" name="action_postcomment" value="Post" >
And I had do add a slash to the end.