“Your best teacher is your last mistake” was said frequently by Ralph Nader, an American political activist, author and attorney. “Your best teacher is your last mistake” was a Nader quote that appeared in a newspaper in April 1972. In 1979, Nader credited his mother for the saying. In a 2009 book, Nader credited his father.
 
   
Wikipedia: Ralph Nader
Ralph Nader (/ˈneɪdər/, Arabic: رالف نادر; born February 27, 1934) is an American political activist, as well as an author, lecturer, and attorney. Areas of particular concern to Nader include consumer protection, humanitarianism, environmentalism, and democratic government.
 
Nader came to prominence in 1965 with the publication of his book Unsafe at Any Speed, a critique of the safety record of American automobile manufacturers in general, and most famously the Chevrolet Corvair. In 1999, a New York University panel of journalists ranked Unsafe at Any Speed 38th among the top 100 pieces of journalism of the 20th century.
 
Nader is a five-time candidate for President of the United States, having run as a write-in candidate in the 1992 New Hampshire Democratic primary, as the Green Party nominee in 1996 and 2000, and as an independent candidate in 2004 and 2008.
 
17 April 1972, The Sun (Baltimore, MD), “Ralph Nader catalogues societal ills for Baltimore audiences” by Earl Arnett, pg. B1:
“Your best teacher is your last mistake.”
   
Google Books
The New Yorker
Volume 49, Part 5
1973
Pg. 62:
“Your best teacher is your last mistake,” he (Ralph Nader—ed.) tells his co-workers over and over. He seldom looks back. When someone tries to draw him out about his past accomplishments, he gives the impression of being uninterested or restless.
 
20 February 1979, Seattle (WA) Times, “Nader: still raging after all these years,” pg. A2, col. 5:
“My mother once told me that your last mistake is your best teacher,” Nader wrote recently.
 
CNNMoney
WHY DUMB THINGS HAPPEN TO SMART COMPANIES YOU’VE HIRED THE SMARTEST PEOPLE AND YOU’RE SPENDING TONS ON R&D AND CUSTOMER SERVICE, YET YOU KEEP BLOWING IT. TIME TO LOOK AT HOW YOU MANAGE BRAINPOWER
(FORTUNE Magazine)
By THOMAS A. STEWART
June 23, 1997
(...)
—You repeat mistakes. “Your best teacher is your last mistake,” Ralph Nader once told me. He, of course, has made a career out of publicizing companies that display this knowledge problem. It’s rampant. Negligence lawyers don’t wear Gucci loafers because companies make mistakes; they wear them because companies make the same mistake twice. The nature of icebergs being what it is, for every million-dollar lawsuit there must be tens of millions lost or wasted from repeated mistakes that are dumb but not tortious.
   
,a href=“http://books.google.com/books?id=btx_bnIbwZcC&pg=PA74&dq=%22your+last+mistake%22+%22+best+teacher%22&hl=en&sa=X&ei=mz5vUvyMHqzi4AOO24HYAQ&ved=0CDUQ6AEwATgK#v=onepage&q=%22your%20last%20mistake%22%20%22%20best%20teacher%22&f=false”>Google Books
The Seventeen Traditions
By Ralph Nader  
New York, NY: Harpercollins Publishers
2009
Pg. 74:
My father used to say, “Your best teacher is your last mistake.” This was a bundle of wisdom we took to heart: Like all children, we made plenty of mistakes, so therefore we had lots of teachers.
 
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Emie
‏@educatetheminds
Just saw this and had to share ‘cause it’s so true and so human- “Your best teacher is your last mistake.”  Author Unknown
8:24 PM - 28 Oct 13