Make sure both your original data and the data you are adding are defined in the correct projection. If the data was defined as the wrong projection than what it was created in, you can get this error. Bare in mind many different types of projections have the words "NAD 1983" in them because that is the datum name, so it is easy to get confused. One common cause of this I have seen is when the projection really uses the 1927 datum.