“I can spell ‘banana,’ but I never know when to stop”

The word “banana” is sometimes difficult to spell, with an extra “n” and “a” often added. An old joke told of a small child who said that he could spell “banana,” but that he “didn’t know where to stop.”
 
The joke dates to at least 1919, when it was possibly coined in a “Mutt and Jeff” comic strip. Adlai Stevenson (1900-1965) used the joke in a campaign speech in 1952.
 
   
15 February 1919, Grand Rapids (MI) Press, pg. 16:
Jeff: THAT’S THE LIMIT. HERE I AM WRITING A LETTER TO ANOTHER AND I GET STUCK ON THE FIRST WORD. I WANT TO TELL HER THAT BANANAS ARE SCARCE HERE IN FRANCE AND THE WORD “BANANA” HAS ME GUESSING!
Jeff: MUTT, HOW DO YOU SPELL “BANANA”? I’M WRITING A LETTER TO MOTHER TELLING HER A BANANA CAN’T BE BOUGHT AT THE FRONT.
Mutt: YOUR IGNORANCE IS REFRESHING. IT STARTS WITH A “B” AND THEN COMES “A”—AND THEN “N-A”—
Jeff: I KNEW THAT. I GOT THAT MUCH WRITTEN DOWN AND THEN I WAS STUCK! I COULDN’T FINISH IT.
Mutt: FINE! NOW LISTEN! THE WORD “BANANA” IS A VERY EASY WROD TO SPELL BECAUSE EVERY OTHER LETTER IN IT IS “A”.
Jeff: I KNOW THAT, TOO, BUT THE TROUBLE IS I DON’T KNOW WHEN TO STOP. B-A-N-A-N-A-N- 
 
8 January 1921, Portsmouth (OH) Daily Times, pg. 12, col. 7:
Repetitious.
We can appreciate the difficulty of the little one who said she knew how to spell “banana” all right, only she didn’t know where to stop.—Boston Transcript.
 
Google Books
Major campaign speeches, 1952
By Adlai Ewing Stevenson
New York, NY: Random House
1953
Pg. 296:
I go from town to town and talk interminably that I am a little like the girl in school who was asked to spell “banana,” and she said, “I can spell banana, but I never know when to stop.”
   
100 Years of Johnny Mercer - Song Database
I Can Spell Banana
Alternate Title: But I Never Know When To Stop
Lyricist: Johnny Mercer
Composer: Geoff Clarkson
Date: 1958