[deltaoneseven]

Agreed, but I would go further to say that the type system and immutability helps in the area you describe as well: "or where you failed to predict some sequence of actions and account for it correctly."

Basically immutability serves to create more invariants and reduce complexity of the program such that it is much more predictable.

I'm advocating exactly your conclusion hence the reason why I specified that immutability is the keyword, not monads and other advanced concepts associated with functional programming.