Visible crew/equipment: The side camera is visible in a couple of shots when the bus has to make the tight right turn, after the bus returns all the wheels to the ground, as it smashes into the cop car. (00:45:20)
Visible crew/equipment: As the end credits are starting, tracks used for a camera dolly can be seen along the left side next to the buildings. (01:50:55)
Visible crew/equipment: When Payne is going crazy on the subway train, and starts shooting his gun at Jack on the roof, in one shot before Jack drops his gun, you can see a camera come in the shot. (01:42:50)
Visible crew/equipment: When Jack is running alongside the subway train trying to find a way in, a camera and its operator can be seen on the window's interior reflection. (01:40:20)
Revealing mistake: When the train is sliding along the ground near the end, there are sparks coming from the underneath (supposedly where the metal scrapes the pavement). Look more closely, and you can see that the sparks are coming from a bit of the train that's not touching the pavement anywhere. (01:44:55)
Continuity mistake: After Keanu commandeers the civilian's car, breaks off the door and jumps to the bus the civilian loses control and heads for the water barrels in front of the divider. Right before he hits you can already see water all over his windshield. (00:38:20)
Continuity mistake: In the last 20 minutes of the movie when Howard Payne is on the train and offers Jack some money, Howard opens the money bag and gets sprayed with purple paint. When Howard climbs on top of the train to fight Jack, all of the paint is gone.
Visible crew/equipment: When the speeding bus takes the really sharp right curve, while it leans over on its left wheels, you can see the crew's tire rig lift under the bus that they used to keep it steady for this stunt. (00:50:10)
Visible crew/equipment: When Jack is talking to Howard Payne from the payphone, you can see the boom microphone's reflection on the phone receiver in several shots. (00:30:00)
Visible crew/equipment: Aside from the mysterious other trooper(s) reflected in the tarpaulin on the roof near the start, as Keanu Reeves runs past it, you can see the clear silhouette and red light of a cameraman filming the scene. (00:11:30)
Revealing mistake: During the rescue of the elevator passengers at the start of the movie, when coaxing the last woman out, there is a shot of the interior of the elevator where you can see the ground through the hole in the elevator floor, even though the elevator is still meant to be on the 22nd floor. (00:16:47)
Continuity mistake: After Jack wounds Harry's leg, Payne makes his escape to the parking garage. Watch as a garbage can appears from nowhere on the left hand side of the door frame in the shot where Payne starts laughing maniacally. (00:22:40)
Continuity mistake: When Jack starts wiggling the train's pole, his hands switch places between shots. (01:46:05)
Continuity mistake: When Jack and Harry are on the top of the building at the start, Jack climbs up a ladder and proceeds to walk along to the crane. In one shot he moves the big gun round to his back, and in the following shot the big gun is still in front of him. (00:11:55)
Continuity mistake: Just before the first bus explodes, Jack leaves the cafe and puts his coffee cup onto the top of his car. When he runs back to his car, the coffee cup is different as well as in a different position. (00:26:25)
Other mistake: As the first bus explodes on its way from the cafe, we can see a blonde pedestrian casually walk past a window with her ears covered. At no point does she even stop to glance at the burning wreck. It seems (from her perspective) as though the explosion were merely an inconvenient loud noise and nothing worth noticing. (00:27:55)
Continuity mistake: At the beginning, when the employees are getting into the elevator, there is a close view of the man at the front pushing the button with his finger, but when it cuts to a wider shot, he is taking his thumb away from the button, instead of his finger. (00:04:35)
Visible crew/equipment: Close to the beginning of the film we see Jack in a coffee shop and his friend walks in. When they both leave and Jack goes to his car, a cameraman is reflected in the car window after the palm tree. (00:26:40)
Continuity mistake: At the beginning of the movie after Jack has hooked up the crane from outside to the lift, there is a shot of the crane outside, and the line going through the vent. If you look closely there is a pipe in front of the vent, yet when the crane goes through the vent, the pipe has vanished. (00:14:20)
Continuity mistake: Near the end of the film when Keanu Reeves and Dennis Hopper are wrestling on top of the subway, if you watch Keanu's left wrist you notice he is wearing a watch. Yet throughout the scene it is obvious that the stunt double has no watch, because in the extreme action scenes, there are no signs of it anywhere on his wrist.
Answer: Ventilation shafts or roof panels.
Toolio