A knock-knock joke involving the terrorist attacks of September 11, 2001 is:
 
“Knock, knock.”
“Who’s there?”
“9/11.”
“9/11 who?”
“You said you’d never forget!”
 
The joke has been cited in print since at least June 2007 and was cited on Twitter on June 19, 2008. A 2009 book called this “the worst knock-knock joke in history.”
     
 
Wikipedia: September 11 attacks
The September 11 attacks (also referred to as 9/11) were a series of four coordinated terrorist attacks by the Islamic terrorist group Al-Qaeda on the United States on the morning of Tuesday, September 11, 2001. The attacks consisted of suicide attacks used to target symbolic U.S. landmarks.
 
Four passenger airliners—which all departed from airports on the U.S. East Coast bound for California—were hijacked by 19 al-Qaeda terrorists to be flown into buildings. Two of the planes, American Airlines Flight 11 and United Airlines Flight 175, were crashed into the North and South towers, respectively, of the World Trade Center complex in New York City. Within an hour and 42 minutes, both 110-story towers collapsed, with debris and the resulting fires causing partial or complete collapse of all other buildings in the World Trade Center complex, including the 47-story 7 World Trade Center tower, as well as significant damage to ten other large surrounding structures. A third plane, American Airlines Flight 77, was crashed into the Pentagon (the headquarters of the United States Department of Defense) in Arlington County, Virginia, leading to a partial collapse in the Pentagon’s western side. The fourth plane, United Airlines Flight 93, initially was steered toward Washington, D.C., but crashed into a field near Shanksville, Pennsylvania, after its passengers tried to overcome the hijackers.
 
Google Groups: alt.buddha.short.fat.guy
Peer Review
Lazarhat
6/4/07
(...)
Here’s a couple of jokes from my Erisian/Discordian friends:
 
Knock, knock.
Who’s there?
9/11
9/11 who?
You said you’d never forget!
 
Forums of Loathing
Really Bad Joke Thread
muley
Sat, Dec 15th, 2007, 01:31 AM
knock knock
who’s there?
9/11
9/11 who?
you said you’d never forget!
 
Google Books
Stop Me If You’ve Heard This:
A History and Philosophy of Jokes
By Jim Holt
New York, NY: W. W. Norton & Company, Inc.
2008
Pg. 78:
Some of these, oddly, take the form of a child’s knock-knock joke (“Knock knock.” “Who’s there?” “9-11.” “9-11 who?” “YOU SAID YOU’D NEVER FORGET!”).
   
Twitter
SocialSoundSystem
‏@SocialSound1982
“Knock knock. Who’s there? 9/11. 9/11 who? You said you’d never forget.” - from dj interview w/ ny mag
5:56 PM - 19 Jun 2008
 
Twitter
Raphael Caffarena
‏@caffarena
A: Knock-knock.
B: Who’s there?
A: 9/11.
B: 9/11 who?
A: You said you’d never forget! Rá!!
11:31 AM - 8 Jul 2008
   
New York (NY) Times
Reflections on That Dreadful Tuesday
By ED PARK SEPT. 6, 2008
(...)
Joke in My In-Box, July 3, 2008

A: Knock, knock.
B: Who’s there?
A: 9/11.
B: 9/11 who?
A: You said you’d never forget!
 
Google Books
The Pocket Guide to Brilliance
By Bart King
Layton, UT: Gibbs SMith
2009
Pp. 19-20:
My own neighbor told me a joke that qualifies as the worst knock-knock joke in history:
 
KNOCK, KNOCK.
WHO’S THERE?
9/11.
9/11 WHO?
YOU SAID YOU’D NEVER FORGET!
   
reddit
Knock knock. Who’s there? 9/11. 9/11 who? (self.Jokes)
submitted May 25, 2014 by jarheadsynapze
I thought you said you’d never forget.