What's fixed:
1.9 Snapshots (15w34c) added "Version: 15w34c" to the world selection screen screen, indicating the last MC version the world was loaded with. Older maps say "Version: unknown." Also If you hover over a world ICON you get the popup message "Don't forget to backup this world before you load this snapshot."

What's still broken:
Unfortunately, There is no popup, over the text area so if you select a world by text, and press the Play button, you never see the warning.

Proposed Solution: Add the same popup message to the "Play Selected World" button when appropriate OR Better, drop the popup and add a message area above the Buttons, to display any warnings, alerts or error messages.