> You can write something using fork-join that works correctly in theory, but expecting it to always work while running on undeterministic hardware is exactly the kind of blind optimism that causes so much software to come down crashing if you look at it wrong.

This seems backwards to me - a fork-join task has no state and can access no other task's state. You can restart it if it fails as many times as you want and the result is always exactly the same. You can't say the same with running an actor twice or sending a message twice - you may get a different result the second time!