__TOC__

== First heading ==

=== subheading ===

== Second heading ==

This give HTML validation error.

If you remove the Second heading, the page validates OK.

If you remove the subheading instead, the page validates OK.

If you remove the <nowiki>__TOC__</nowiki>, or replace it with <nowiki>__NOTOC__</nowiki> in articles with more headings, the page validates OK.