In the "Select World" browser klicking a world immediately plays it and not just selects it to be renamed, deleted, etc.
How to produce:
Loading a world and klick "save and quit to title" as soon as possible (while chunks still load)
Now if you klick on a world in the "Select World" browser it immediately plays that world and doesn't select it.
This is fixable by minimizing the window.
Can confirm. This happened to me the first time I started up 14w17a.