When Debbie Renold's character is recording her voice to go over Lina Lamont's, the line you hear her say is "Our love will last 'till the stars turn cold." When they play the "talkie" at the end of the film, it is actually Carry Grant's character that says that line!
Um, Cary Grant isn't in the movie; I think you mean Gene Kelly. And I don't think this is a slip up. I think in the fictional movie they're making, Gene's repeating a line Debbie/Lina said earlier. Kinda like a catchphrase or something.
Sorry to tell you, but Cary Grant was not in Singin' in the Rain. It was Gene Kelly. There are completely different actors- Kelly can dance and sing, and Grant is more of a suave sophisticate.
Um, have you actually seen this movie Carey Grant doesnt even make a cameo appearence. Also Debbie Reynolds character was probably recording that line so that Lena could practice talking properly.