When John and Jane are questioning their former apparent target near the end of the film, look carefully at the guy's shirt - it's the logo from another Brad Pitt film, Fight Club.

A subtle clue that Brad Pitt is a secret agent/spy: he drinks vodka martinis with two olives - shaken, not stirred - just like that other famous spy, James Bond. Submitted by Xofer

Nicole Kidman was supposed to star along side Brad Pitt, but had to pull out due to other commitments. When she did this, Brad also pulled out but later re-joined when Angelina Jolie was cast.