Plot hole: When Cary Grant is chasing after Audrey Hepburn where she has to meet Walter Matthau outside the Paris Opera he shouts "Reggie stop - that man is Carson Dyle". Tex recognised him, that's why he wrote Dyle on the carpet. But Cary Grant hadn't gone back into the room at the hotel, so he couldn't even have known Tex was dead.
Plot hole: When Walter Matthau is interviewing Audrey Hepburn in his "office," she is looking at a picture. She identifies various faces in the picture. Matthau correctly identifies the person without knowing which face she was asking about. Other than her husband, he could not know which of the other three faces she was studying.Noman