A popular engineering joke is:
   
Q: What is the difference between mechanical engineers and civil engineers?
A: Mechanical engineers build weapons; civil engineers build targets.
 
The joke has been cited in print since at least 1998 and is of unknown authorship.
 
         
25 March 1998, The Facts (Clute, TX,), “Joke of the Day,” pg. 1C, col. 3:
Q. What is the difference between mechanical engineers and civil engineers?
A. Mechanical engineers build weapons, civil engineers build targets.
 
A Prairie Home Companion (April 18, 1998)
The Third Annual Joke Show
(...)
The difference between mechanical engineers and civil engineers is that mechanical engineers build weapons, civil engineers build targets.
 
Google Groups: alt.tasteless.jokes
What is the difference between Mechanical Engineers and Civil Engineers?
G.P.
10/10/98
Mechanical Engineers build weapons,
Civil Engineers build targets.
     
Google Books
Midwest Engineer
Volumes 51-52
1998
Pg. 44:
What’s the difference between mechanical engineers and civil engineers?
Mechanical engineers build weapons; civil engineers build targets.
 
Google Books
Practically Speaking:
A Dictionary of Quotations on Engineering, Technology .
By C.C. Gaither and Alma E Cavazos-Gaither
Philadelphia, PA: Institute of Physics Publishing
1999
Pg. 87:
What is the difference between Mechanical Engineers and Civil Engineers?
Mechanical Engineers build weapons; Civil Engineers build targets.
Source unknown
   
Google Books
Speaking of Science:
Notable Quotes on Science, Engineering, and the Environment
By Jon Fripp, Michael Fripp and Deborah Fripp
Eagle ROck, VA: LLH Technology Publishing
2000
Pg. 79:
Q: What is the difference between mechanical engineers and civil engineers?
A: Mechanical engineers build weapons, civil engineers build targets.
—Anonymous
   
Jokes about engineers
Updated 3 May 2001
What is the difference between Mechanical Engineers and Civil Engineers?
 
Mechanical Engineers build weapons,
Civil Engineers build targets.
 
August 2001, Civil Engineering : Magazine of the South African Institution of Civil Engineering, “Engineers” by Johan de Koker, pg. 46:
Comprehending engineers - Take one: What is the difference between mechanical engineers and civil engineers?
Mechanical engineers build weapons; civil engineers build targets.
 
6 August 2001, Florida Today (Melbourne, FL), “Relax; it’s not a new mosquito” by Milt Salamon, pg. 8:
—- LESSONS from “Understanding Engineers 101,” a basic course e-mailed to us by Don Jordan of Melbourne:
 
1. To the optimist, the glass is half-full. To the pessimist, the glass is half-empty. To the engineer, the glass is twice as big as it needs to be.
 
2. The difference between Mechanical Engineers and Civil Engineers?
Mechanical Engineers build weapons, Civil Engineers build targets.
 
Google Books
Engineering, Business & Professional Ethics
By Simon Robinson, Ross Dixon, Christopher Preece and Krisen Moodley
New York, NY: Butterworth-Heinemann
2007
Pg. 69:
What’s the difference between mechanical engineers and civil engineers?
Mechanical engineers build weapons
Civil engineers build targets
 
Google Books
Gaither’s Dictionary of Scientific Quotations (Second Edition)
Edited by Carl C. Gaither and Alma E. Cavazos-Gaither
New York, NY: SPringer
2012
Pg. 632:
What’s the difference between Mechanical Engineers and Civil Engineers?
Mechanical Engineers build weapons; Civil Engineers build targets.
Source undetermined