[- - - ATOC - - -] (spaces added so you can see the hyphens)
[- - - TAG:h2 - - -]
Unfortunately, some time in the past week WordPress has decided that when I type in 3 hyphens what I really wanted was an mdash. (This issue may have been caused by the WP 4.0.1 security update on Thursday.)
- - - becomes ---
Yes, that is a common typography trick, but WP didn't used to muck around with shortcodes. Now WP is changing the text inside the shortcode and that broke the TOC.
I have already contacted the developer of the TOC plugin, and I am about to go complain to Automattic, but if anyone knows how I can patch this myself I would really appreciate it.
Update: A developer acquaintance of mine pointed me at a support thread on WordPress.com which explained that the broken shortcode was a known issue with that recent WP update and was the result of the plugin not being programmed correctly. So I guess I will have to wait for the Plugin's developer to update.