“A clear conscience is usually the sign of a bad memory”

“A clear conscience is usually the sign of a bad memory” is a jocular line that has been cited in print since at least 1998. Authorship is unknown.
 
American comedian Steven Wright has been infrequently credited with the saying since at least 1999, but there is no documentary evidence that the saying is his.
 
 
MARY BAKER EDDY LETTER
Letter No. Eleven, October 19, 1998
(...)
Someone once said that a clear conscience is usually the sign of a bad memory.
 
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arrow rests
Triad
11/9/98
What would be a good rest to buy for hunting,something priced resonable,and quiet. I have looked at the TM Hunter..Is this a good
rest.—
A clear conscience is usually the sign of a bad memory
 
Google Groups: alt.rock-n-roll.metal.motley-crue
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1/30/99
(...)
“A clear conscience is usually the sign of a bad memory”
 
A Prairie Home Companion - Jokes 1999
March 16, 1999
(...)
A clear conscience is usually the sign of a bad memory.
Rita Gillmer, Baton Rouge,, LA
   
Steven Wright Quotes
Updated 6/21/99
A FEW WORDS FROM THE VISIONARY STEVEN WRIGHT:
(...)
A clear conscience is usually the sign of a bad memory.
   
Google Books
The Mammoth Book of Humor
By Geoff Tibballs
New York, NY: Carroll & Graf
2000
Pg. ?:
[1,859] A clear conscience is usually the sign of a bad memory.
 
Google Books
Echoes from the Poisoned Well:
Global Memories of Environmental Injustice
Edited by Sylvia Hood Washington, Heather Goodall and Paul Rosier
Lanham, MD: Lexington Books
2006
Pg. 97:
A clear conscience is usually the sign of a bad memory —Anonymous
 
Google Books
Sister Mine:
A Novel
By Tawni O’Dell
New York, NY: Shaye Areheart Books
2007
Pg. 286:
“My dad used to say, ‘A clear conscience is usually the sign of a bad memory.’”
 
Google Books
Of Wit N Humour
Compiled by Vincent Thnay
Lulu Press, Inc. (lulu.com)
2015
Pg. ?:
A clear conscience is usually the sign of a bad memory. Steven Wright