Agreed. If you discover something is off, just abort early, don't try to save it.

One classic exploit I've seen being possible multiple times is when you allow a subset of html and remove other tags. A naive approach will for instance remove <script>. But what happen if I then send in <scr<script>ipt>? If it only does one pass, I fooled it. Instead, just assume the input is done in bad faith when it's encountered and crash, instead of kneading it to something acceptable.