Upgrade to 2.7 stuck in 'upgrade database now' loop with undeletable themes

While upgrading to 2.7.1 from 2.6.2+, I came across this loop which can only be removed by manually deleting rows from the database. I experienced this on three upgrades on different servers.

Steps to reproduce (more or less, as I didn't write them down at the time but a short while after):

1. Upgraded from 2.6.2+ to 2.7. No problems encountered.

2. Clicked Notifications, clicked 'Check for updated plugins'. It lists the following themes (only one at this point):

Themes

• Simple (Missing from disk)

I click 'Uninstall' next to Simple, then Continue.

3. Plugin is uninstalled without problems.

4. Page redirects to Plugins Check screen, showing 29 plugins requiring my attention. Some are missing from disk (due to upgrade: this is expected), btu most are themes 'to be deleted'.

5. Click 'Upgrade Moodle database now'.

6. Page redirects to Plugins Overview: 'to be deleted' plugins are still present but 'disabled'.

7. Uninstalling one of the themes (e.g. Afterburner) 'succeeds'.

8. I am taken back to #4, but now there are only 28 plugins requiring my attention.

9. This can continue until there are only two themes left, 'standard' and 'standardold', which cannot be deleted.

To remove the final two themes from the database (as they definitely don't exist on disk):

Look in 'mdl_config_plugins', for an entry called 'theme_standard' in the 'plugin' column, and delete it. Same for 'theme_standardold'.

Reporting as per poltawski's request: https://twitter.com/SDCMoodle/status/485051398934249472