Although incorrectly believed to be a line from the cult movie Casablanca by many people, this line does not appear in the original movie. Both Ingrid Bergman and Humprey Bogart say "Play it, Sam" at separate occasions, but the fame of the line "Play it again, Sam" is likely due to the fact that it appears in the movie "A night in Casablanca" starring the Marx brothers.
The same phenomenon can be observed with "Hamlet" and the scene that specifically does not contain Hamlet with a skull in his hand declaring that "to be or not to be, that is the question".