I think I found a bug in the Service Result/Custom Error section.

If you accidentally rename a line's Customer Error Description to "Success" and you already have one called "Success", you then can no longer change the Custom Error Description or even delete the line. You're stuck with two "Success" lines.

It looks like the code that disallows the deleting/changing of "Success" should allow it, as long as there will be at least one "Success" line remaining after the delete/change.