When you have the last snapshot instaled and ready to play, the launcher says "Ready to update & play Minecraft" instead of "Ready to play Minecraft"
That only happends with the last snapshot (13w37b) only.
forgive me if I have a bad English. Im from Argentina.
Duplicate of MCL-1516, please use the search function to see if your bug has already been submitted. Currently over 53% of tickets are being resolved as duplicate.