When the slipstream device is used on the aircraft, as we see the bullet leaving Sarah's chest, there are a few papers flying through the air behind the bullet. The papers fly by the screen while the bullet is still going backwards towards the gun it was fired from. This would indicate that the papers were flying faster than the bullet, which, given the physics of the scene, is next to impossible.
Following the last use of the device, Stewart is back in the bank talking to the teller again. When he finishes, he just casually walks out. While the FBI agent had looked outside and saw the robber van driving away, Stewart didn't have that information and still should have thought that the robbery was about to take place again.