Erlang 20 and RabbitMQ.
RabbitMQ versions before 3.6.11 will not work correctly with OTP-20
When upgrading from OTP-19.x(or earlier) to OTP-20 all the persistent data will be permanently lost!
Although it's possible to run RabbitMQ with OTP-20 from scratch, there will be crashes in queue mirroring and management API.