Jokes about the September 11, 2001 terrorist attacks on New York City’s World Trade Center are numerous, although not politically correct. One such politically incorrect joke is:
   
Q: Why were the twin towers sad?
A: Because they ordered pepperoni pizza and got plane (plain).
   
The joke was posted on Twitter on May 9, 2013 and on Reddit—Jokes on August 20, 2015.
 
   
Wikipedia: World Trade Center (1973-2001)
The World Trade Center was a large complex of seven buildings in Lower Manhattan, New York City, United States. It featured landmark twin towers, which opened on April 4, 1973, and were destroyed in the September 11 attacks, with 7 World Trade Center collapsing later that day due to the damage it suffered when the twin towers collapsed that morning. The other buildings in the complex were severely damaged by the collapse of the twin towers, and their ruins were eventually demolished.
 
At the time of their completion, the “Twin Towers” — the original 1 World Trade Center, at 1,368 feet (417 m); and 2 World Trade Center, at 1,362 feet (415 m) — were the tallest buildings in the world.
         
Twitter
Yahstrada 〽️us
‏@Yahstradamus
Why were the twin towers sad?
They asked for pepperoni and got plane
10:49 AM - 9 May 2013
 
Twitter
Mitchell Christy
‏@Barefoot__Panda
Q: Why were the twin towers sad?
A: Because they ordered pepperoni and got plane
11:12 PM - 9 May 2013
 
Twitter
Vile Jokes
‏@VileJoke
Why were the twin towers sad on 9/11?
They ordered pepperoni pizza but got plane.
http://vilejoke.com #offensive #vile
RETWEET
12:07 PM - 30 Jan 2014
 
Reddit—Jokes
Why were the twin towers sad?
submitted August 20, 2015 by lesnable
They ordered pepperoni but got plain.
 
Reddit—Jokes
Why were the twin towers sad?
submitted February 1, 2017 by Oblivious__Oblivion
Because they ordered pepperoni pizza and got plane.