Online Server Chat loaded in Singleplayer

I initially opened MC and connected to play.skycade.net but then clicked cancel as I decided to play singleplayer instead. 

I got back to the main menu with no problem and loaded my recent singleplayer world, but the first time it loaded it was super glitchy, just sky everywhere I looked. I exited and tried again and the world loaded correctly, but I am seeing the chat from skycade in my singleplayer world. This seems to be live as it is updating too and not just a one off grab of the chat. 

AS I was typing this the skycade SMP server I normally play on reset, and now I am IN skycade. This will make me sound utterly crazy now,  but the attached screenshot shows that the window says singleplayer, and I was 100% in the singleplayer world I setup as the house I have there is new. 

I have set this to private as I am not sure if this could be exploited in some way.

Additionally when clicking save and quit Minecraft crashed.