“In theory, there is no difference between theory and practice. But, in practice, there is.”

Entry in progress—B.P.
 
Wikipedia: Jan L. A. van de Snepscheut
Jan L. A. van de Snepscheut (1953 – 1994) was a computer scientist and educator.
Unsourced
In theory, there is no difference between theory and practice. But, in practice, there is.
. This has also been attributed to Yogi Berra.
. This has also been attributed to Chuck Reid.
     
Google Books
An introduction to the art and science of programming
By Walter J. Savitch
Menlo Park, CA: Benjamin/Cummings Pub. Co.
1986
Pg. 366:
In theory, there is no difference between theory and practice; but in practice, there is.
 
Google Books
Pascal, an introduction to the art and science of programming
By Walter J. Savitch
Redwood City, CA: Benjamin/Cummings Pub. Co.
1991
Pg. 507:
In theory, there is no difference between theory and practice, but in practice, there is.
 
Google Books
Parallel multigrid waveform relaxation for parabolic problems
By Stefan Vandewalle
Stuttgart: Teubner
1993
Pg. 248:
In theory, there is no difference between theory and practice. But, in practice, there is.
—/ bin/ fortune.
   
Google Books
Philosophical perspectives on bioethics : [essays presented at sessions of a 1993 - 94 seminar series]
Edited by Leonard W Sumner
Toronto: Univ. of Toronto Press
1996
Pg. 133:
An unknown wit stated it perfectly: ‘In theory, theory and practice are the same thing. In practice, they’re not.’
   
Google Books
Applying use case driven object modeling with UML:
An annotated e-commerce example
By Doug Rosenberg and Kendall Scott
Boston, MA: Addison-Wesley
2001
Pg. 1:
One of our favorite quotes is, ‘The difference between theory and practice is that in theory, there is no difference between theory and practice, but in practice, there is.”