After Austin buys himself and Piper a fully loaded hot dog with all the massive toppings, he spills it on her sketchbook. They cut to his face, and then back to the view of the sketchbook once again. Austin picks it up and the drawing is spotlessly clean. There is no way all the ketchup and toppings could have magically disappeared.
While the characters are walking through parts of BTC, in the background they show the Twin Towers. This movie was made well after 9/11.