Visible crew/equipment: When Anderton visits his wife after the auto plant scene, the camera pans around the car and the camera operator is clearly seen in the reflection on the bumper. (01:57:05)
Visible crew/equipment: When Lamar shoots Witwer it shows the the bullet's point of entry. Looking closely at his shirt, you can see the outline of the square/rectangular blood pack. (01:48:50)
Visible crew/equipment: In the scene where Anderton is going to kill Crow, the camera angle changes to behind Crow. In the window behind him you can see the demolitions charge used to create a bullet hole and crack the glass. (01:44:05)
Visible crew/equipment: When John and Agatha come to Rufus T Riley's place there is a low camera view of them walking through a doorway as Rufus starts saying about Anderton getting 'a little jerky for himself'. To the left of the doorway the crew are reflected going backwards filming the action.
Visible crew/equipment: After the scene in the car factory when Anderton drives up to Hineman's place, he parks his car and opens the door. When he opens the door, you can very easily see a cameraman bent down reflected in the car door. (00:53:10)
Visible crew/equipment: When John Anderton shoots Leo Crow, the camera mounted on a crane can be seen reflected on the building's glass in the outside shot just a second before the man defenestrates himself.
Visible crew/equipment: When Anderton is about to enter the Lexus factory he runs into the cops' car. When the passenger door opens, you can see the reflection of a crewmember. (00:49:35)
Visible crew/equipment: When John Anderton is walking around the hotel room before meeting Leo Crow, just before he spots the photos on the bed, he glances over towards the direction of the bed - if you look to the left of him, you can make out a cameraman's face and hand where he is holding a camera in the reflection of the glass window.
Answer: Because subverting the expectations of the viewer makes it more interesting. The audience (and potentially John) are set-up to expect bad things, which don't happen. Once the "bad thing" happened, the suspense would be gone and everyone could relax. Expecting something bad but knowing when it might happen maintains the tension.