because we can miscalculate an "ought" or anything else.

One way to miscalculate an "ought" is the same way that we can miscalculate an "is" -- e.g. lack of information, erroneous knowledge, false understanding of how to weigh data, etc.

And also, because people aren't perfectly self-aware, we can mistake mere habits or strongly-held preferences to be the outputs of our moral algorithm -- same way that e.g. a synaesthete might perceive the number 8 to be colored blue, even though there's no "blue" light frequency striking the optical nerve. But that sort of thing doesn't seem as a very deep philosophical problem to me.