The difference between [1, "two"].sum and 1 + "two" is that in the second case you are passing "two" to 1's :+ method, that's an argument error, it doesn't depend on 1's state. In the first case you are not giving any argument to the method, so you shouldn't get an error. From the point of view of the array's user the array is now in an invalid state and that's not something the array should have allowed to happen.