Always have legal counsel review any such correction or retraction before it is broadcast. Given that tone, it is perhaps not surprising then, that corrections are heard infrequently. Morning Edition does it every two to three weeks. Saturday , not at all. Producers use it as a device to address listeners' concerns and to run corrections.
Producers also use it as a timing device if there is a two or three minute hole in the show.
Whenever NPR airs a correction, it should be appended to the original transcript to make sure the record is amended as newspapers do. One solution would be to use the NPR Web site. Correcting major errors seems unquestioned as a matter of policy. But they represent an important principle. NPR needs to ensure that all its programs respond to listeners' concerns in a timely manner either in the program, or on the Web site whenever a mistake has been aired.