WordPress 3.9 has an upgraded version of TinyMCE that a lot of plugins aren’t compatible with yet, since some of them add their own buttons/functionality to the “Kitchen Sink”. Some themes, including Largo, interact with it too.
The most common issue is that the post “Visual” editor doesn’t work as it should.
This is what you’ll need to do to fix the issue:
1. Download the latest Largo from: http://blargo.
2. Disable the “Media Credit” plugin if it isn’t already disabled