MySQL mistake is a wake-up call on open source ownership
There was a moment of panic in the open source community this week when a developer on the MariaDB fork of MySQL discovered that Oracle had quietly changed the license on all the man pages for MySQL from GPL to a restrictive proprietary license two months earlier. Prompted by the bug report, Oracle's staff quickly discovered that an error had been made in the build system and promised to immediately undo the change and restore the GPL to all of MySQL. Problem solved!
All the same, the incident was a wake-up call to many. Although there's no reason why they should, and they have promised not to do so, Oracle could change the license for MySQL, or indeed any of the open source projects they own, at any time without notice. They are able to do this since, unique among the rest of the open source community around each project, they are not themselves bound by the open source license.
This unique power exists in turn because Oracle owns the entire copyright to MySQL, even to parts of it they have not themselves written. Why is that? It's because all contributors to the code have to sign a "contributor agreement" assigning ownership of the copyright to Oracle. They are not alone in this. Sun before them used contributor agreements to get full source ownership, and many other projects do the same.