“An expert is a man who has made all the mistakes which can be made in a narrow field” is a jocular definition of an expert that has been printed on many images. The quotation has been credited to Danish physicist Niels Bohr (1995-1962). Life magazine quoted physicist Edward Teller (1908-2003) on September 6, 1954:
 
“He (Edward Teller—ed.) likes to quote the dictum of Niels Bohr, the great Danish physicist,  that, ‘An expert is a person who has found out by his own painful experience all the mistakes that one can make in a very narrow field.’”
 
The book Murphy’s Law 2000 (1999) called it “BOHR’S LAW: An expert is someone who has made all of the possible mistakes in a very narrow field of study.”
 
“An expert is a person who has made every possible mistake in a small field of study…. ~Anonymous” was posted on Twitter by Cool Funny Quotes on November 28, 2013.
 
         
Wikiquote: Niels Bohr       
Niels Henrik David Bohr (7 October 1885 – 18 November 1962) was a Danish physicist. He received the Nobel Prize for Physics in 1922 for his contributions which were essential to modern understandings of atomic structure and quantum mechanics.
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An expert is a person who has found out by his own painful experience all the mistakes that one can make in a very narrow field.
. As quoted by Edward Teller, in Dr. Edward Teller’s Magnificent Obsession by Robert Coughlan, in LIFE magazine (6 September 1954), p. 62
. Variant: An expert is a man who has made all the mistakes which can be made in a very narrow field.
As quoted by Edward Teller (10 October 1972), and A Dictionary of Scientific Quotations (1991) by Alan L. Mackay, p. 35
 
Google Books
6 September 1954, Life magazine, “Dr. Edward Teller’s Magnificent Obsession” by Robert Coughlan, pg. 62, col. 1:
He likes to quote the dictum of Niels Bohr, the great Danish physicist,  that, “An expert is a person who has found out by his own painful experience all the mistakes that one can make in a very narrow field.”
         
Google Books
The Log Analyst
Society of Professional Well Log Analysts
Volumes 15-16
1974
Pg. 136:
Nor may we count on obtaining the services of experienced people, (Experience being “the knowledge of a lot of things one should not have done”) nor of experts (An expert being a person who has made all possible mistakes in a very narrow field), nor may we call on consultants in order to share the blame for failures.
 
17 September 1978, Los Angeles (CA) Times, “Nixon Fellow Dr. Edward Teller, 70 Has No Nuclear Regrets” by Robin Heffler, pt. 10, pg. 2, col. 6:
“An expert is a person who through his own person experience has found all the mistakes one can commit in a very narrow field,” he said enunciating each phrase and gesturing for a reporter to take down all his words.
 
Google Books
Reminiscences of Los Alamos 1943-1945
Edited by Lawrence Badash, J.O. Hirschfelder and H.P. Broida
New York, NY: Springer
1980
Pg. 81:
As Einstein said, “An expert is a person who has made every mistake once.”
     
Google Books
Best’s Review
Volume 82
1981
Pg. 84:
A WORLD-FAMOUS engineer once testified in court that he would not object to being called an expert if the litigants permitted him to define an expert as “one who had made almost all possible mistakes in a narrow field of endeavor, and remembered most of them.”
   
Google Books
Murphy’s Law 2000:
What Else Can Go Wrong in the 21st Century!
By Arthur Bloch
Los Angeles, CA: Price Stern Sloan
1999
Pg. 13:
BOHR’S LAW: An expert is someone who has made all of the possible mistakes in a very narrow field of study.
 
Google Groups: alt.comp.hardware.pc-homebuilt
Help - file copy program with verification ?
Dirk
5/26/99
(...)
“An expert is someone who has made every possible mistake in a very narrow field.” N. Bohr
   
Google Books
I Fail to Miss Your Point
By Jim O’Bryon
Xulon Press (XulonPress.com)
2007
Pg. 236:
An expert is a person who has made every possible mistake in a very narrow field of endeavor. (Dr. Edward Teller, Lawrence Livermore National Labs)
 
13 February 2008, Tampa (FL) Tribune, “Bridge” by Frank Stewart, Flavor sec., pg. 7, col. 1:
Physicist Niels Bohr said that an expert is someone who has made every possible mistake in a narrow field of expertise.
   
Twitter
Christian Payne‏
@Documentally
Replying to @vascellari
@vascellari “An expert is someone who has made every mistake in their field.” Who is even doing it hard enough? Not the online marketers..
2:52 AM - 20 Dec 2008
   
Twitter
Jim Apple‏
@5Minutes10
Replying to @corneliusficht
@corneliusficht Neils Bohr had it that an expert is someone who has made every possible mistake in a narrow field of endevour.
5:34 PM - 11 May 2009
   
Google Books
Brilliant Decision Making:
What the best decision makers know, do and say
By Robbie Steinhouse
Harlow, UK: Pearson Education Limited
2012
Pg. ?:
‘An expert is someone who has made every mistake possible in a very narrow field.’
Niels Bohr, Danish physicist and Nobel Prize winner
 
Twitter
Cool Funny Quotes‏
@CoolFunnyQ
An expert is a person who has made every possible mistake in a small field of study…. ~Anonymous http://www.coolfunnyquotes.com/author/anonymous/expert-mistake/ …
10:47 AM - 28 Nov 2013
   
Twitter
Ryann‏
@itsmeeeRyan
An expert is a person who has made every possible mistake in a small field of study.
9:56 AM - 29 Nov 2013
     
Google Books
Space in Mind:
Concepts for Spatial Learning and Education
By Daniel R. Montello, Karl E. Grossner and Donald G. Janelle
Cambridge, MA: The MIT Press
2014
Pg. 281:
Niels Bohr saw an expert as “a person who has found out by his own painful experience all the mistakes that one can make in a very narrow field” (Coughlan 1954, p. 62). Werner Heisenberg offered a positive gloss: “An expert is someone who knows some of the worst mistakes that can be made in his field, and how to avoid them” (Capri 2011, p. 181).
 
Google Books
Niels Bohr, 1913-2013: Poincaré Seminar 2013
Edited by Olivier Darrigol, Bertrand Duplantier, Jean-Michel Raimond, and Vincent Rivasseau
Birkhauser
2016
Pp. 43-44:
What is an “expert”> Most of us would say that it is one who has absorbed all the world’s knowledge in a certain field. Niels Bohr’s formulation was: An expert is one who, from personal, painful experience, knows a little bit about the most serious mistakes that can be committed even within the smallest area.
 
Twitter
Stacey Myers‏
@staceylmyers
An expert is a person who has made every possible mistake in a small field of study. - Anonymous
12:50 PM - 9 Feb 2018