It's called debugging. They code, they compile, they test. They code, they compile, they test. They find problems. Plenty of them.
Architects test, too.
They start with reviews with the client to see if their rough sketches are filling the client's needs. They do this multiple times, until the design begins to work. At least the good ones do. Everything seemed perfect until the costumes arrived for the premiere dress rehearsal. There was nowhere to put them.