Although WP said the update was installed successfully, when I went back to the Plugins section, it said that I still had 4.5 installed and an update was available. I also noticed that any page/post with tabs in it now has “1_tabs” appearing at the top of the page, even though it’s not in the page content.
I deactivated and deleted the plugin, re-downloaded 4.6 from this site, and re-installed it. It still reports as having 4.5 installed, although WP – for now at least – no longer says there’s an update available, and the pages with tabs still have “1_tabs” at the top of them. Example: http://paranoimia.uk/mika