Accidentally Upgrading to Master (4.1dev ALPHA)

A recent comment on my channel reported an error message that can result from upgrading to the "master" branch of Moodle. Where it's not the end of the world, this is irreversible and is rather easy to do.
Here is the message:
ERROR!!! The code you are using is OLDER than the version that made these databases!
If you ever find that you have accidentally upgraded to the development version, here's why it happened and what you can do to retrieve your courses and data.