“The last line of the national anthem is often thought to be ‘Play ball!’”

“The Star-Spangled Banner” is the national anthem of the United States. The anthem has been regularly played at the start of baseball games since World War II. A frequent joke is that the last line of the national anthem is “Play ball!”
   
“After all, isn’t the last line of our national anthem ‘Play ball?’” was cited in 1984.
 
   
Wikipedia: The Star-Spangled Banner
“The Star-Spangled Banner” is the national anthem of the United States. The lyrics come from “Defence of Fort M’Henry”, a poem written in 1814 by the 35-year-old lawyer and amateur poet Francis Scott Key after witnessing the bombardment of Fort McHenry by British ships of the Royal Navy in the Chesapeake Bay during the Battle of Fort McHenry in the War of 1812.
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In 1916, President Woodrow Wilson ordered that “The Star-Spangled Banner” be played at military and other appropriate occasions. The playing of the song two years later during the seventh-inning stretch of Game One of the 1918 World Series, and thereafter during each game of the series is often cited as the first instance that the anthem was played at a baseball game, though evidence shows that the “Star-Spangled Banner” was performed as early as 1897 at opening day ceremonies in Philadelphia and then more regularly at the Polo Grounds in New York City beginning in 1898. In any case, the tradition of performing the national anthem before every baseball game began in World War II.
       
19 April 1984, Trenton (NJ) Times, “Ode to the real thing: baseball” by Harry Sayen, pg. B3, col. 4:
After all, isn’t the last line of our national anthem “Play ball?”
 
Google News Archive
5 January 1989, Bangor (ME) Daily News,  “Predictions for the new year” by Donald Kaul, pg. 10, col. 5:
NOVEMBER—President Bush announces he’s changing the last line of the national anthem from “Play Ball” to “Don’t Worry, Be Happy.”
   
Google Books
Playing with God:
Religion and Modern Sport
By William J. Baker
Cambridge, MA: Harvard University Press
2007
Pg. 108:
To a generation of Americans who have grown up with televised sport, it seems that “Play ball!” has always been the last line of the National Anthem.
 
Nothing could be farther from the truth. The alignment of patriotism with sport is essentially an “invented tradition” born in the nineteenth century when nationalistic holidays, flags, anthems, pledges of allegiance, and parades first flourished throughout the western world.
 
Twitter 
Joe Heuer
‏@RockandRollGuru
Play ball: the last line of the national anthem. #SillySun
5:20 PM - 7 Feb 2010
 
Twitter
Jim Gaffigan
‏@JimGaffigan
Everyone thought the last line of the National Anthem was “Play ball”, right?
10:29 PM - 18 Oct 2011
 
Twitter
Sasha Ericsson
‏@Sasha_Ericsson
Everyone else thought the last line of the National Anthem was “Play ball”, right?
3:17 PM - 23 Aug 2014
 
Twitter
Lynn Berenbaum
‏@lynnlovestennis
I just recently found out that the last line to the American National Anthem is not “Play Ball!” #whoknew #joke
5:21 PM - 1 Feb 2015