2 worlds where such incident has happened where the following occurs
Friends who has been able to join world is suddenly no longer join 
They are stuck on ''locating server'' screen
There character is displayed  in the world but you are unable to interact with them 
User can never join world again but can join other worlds made by same person
Making copies of the world don't work either

I would very much appreciate it if they patch this ASAP thank you.