In the beta if you do unknown the game decides to make all AI be deactivated, you can still walk and break blocks, BUT these broke: block picking (middle cursor button), most particles and animations (If you break a block it just disapears without particles and sounds). There is probably more, I just looked at it and rejoined, but it continued like that until I closed the game. (Note for mojang: Some numbers about "server" were frozen when this happened.)

How to reproduce:

unknown