Some objects which pre-dated 9/11/01, taking on signficance only afterward:
(1) A postage stamp
(2) A 1979 ad for Pakistani Airlines (click to enlarge)
(3) An ad that ran a few days earlier
(4) Tourist coffee mug:
UPDATE: And then there’s this. In March of 2001 Fox TV aired Episode One of a new show spun-off from The X-Files. In the first episode of this new show, The Lone Gunmen, an inside faction of the government posing as terrorists hijacks a 727 by remote control. They do this because the cold war is over they need an excuse for war to increase arm sales. They target the World Trade Center, but their plot is foiled at the last minute.