A recent post left a <code> tag unclosed, affecting the formatting of later posts in the thread. I vaguely recall that a similar fix was made earlier, perhaps for <pre>?

Broadly, any HTML that Markdown allows should be checked and fixed up by the Markdown to HTML processor. I'd guess someone could have a lot of fun with unclosed <table> tags, for example.

It may suffice simply to increment a count any time such an opening tag is found, decrement it when the closing tag is found, and then blindly generate closing tags when the count is > 0 at the end of the document.