“A well-adjusted person is one who makes the same mistake twice without getting nervous” means that the well-adjusted person is not afraid to make make mistakes—and he or she usually does make them. The quotation is sometimes attributed to Alexander Hamilton (1757-1804), but this is without documentation and is highly improbable. Jane Heard has been credited since at least 1989, but it’s not known who this person is or when she said it.
 
“A well-adjusted person is one who makes the same mistake twice without getting nervous” was cited in The American Mercury magazine in 1960. This appears to be the source; the saying was frequently cited in the 1960s.
 
   
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The American Mercury
Volume 91
July-December 1960
Pg. 118:
A WELL-ADJUSTED PERSON is one who makes the same mistake twice without getting nervous.
 
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Coronet
Volume 49
1960
Pg. 44:
A well-adjusted person is one who makes the same mistake twice without getting nervous. —American Mercury
 
17 August 1961, Ada (OK) Weekly News, “Strayed From the Herd” by Connie Nelson, pg. 4, col. 2:
A well adjusted person is one who makes the same mistake twice without getting nervous.
 
12 July 1965, Greensboro (NC) Record, “Jest a Minute,” pg. 1, col. 4:
A well adjusted person is one who makes the same mistake twice without getting nervous.
 
Google News Archive
4 March 1967, Rochester (IN) Sentinel, “Hap Hazard’s THIS and THAT From OUT the HAT,” pg. 2, col. 6:
ADJUSTED — A well adjusted person is one who makes makes the same mistake twice without without getting nervous. is more
 
20 July 1989, Marietta (GA) Daily Journal, pg. 7B, col. 1:
Yesterday’s Cryptoquote: A WELL-ADJUSTED PERSON IS ONE WHO MAKES THE SAME MISTAKE TWICE WITHOUT GETTING NERVOUS.—JANE HEARD
 
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Big Wisdom (Little Book):
1,001 Proverbs, Adages, And Precepts To Help You Live a Better Life
By Thomas Nelson Publishers
Nashville, TN: W Publishing Group
2005
Pg. 9:
A well-adjusted person is one who makes the same mistake twice without getting nervous. —Author Unknown
     
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Thoughts For Meaningful Life
By Pano George Karkanis
Bloomington, IN: AuthorHouse
2008
Pg. 14:
A well adjusted person is one who makes the same mistake twice without getting nervous about it (Jane Heard).
 
Google Books
Quote Junkie Funny Edition
by Todd Hagopian (Hagopian Institute)
Charleston, SC : Hagopian Institute
2012
Pg. 5:
A well adjusted person is one who makes the same mistake twice without getting nervous.
Alexander Hamilton