Revealing mistake: The whole structure surrounding the elevator appears shiny and spotless, with the wires lacking a single spot of grease, revealing it's a set. (00:04:25)
Revealing mistake: When Harry and Jack arrive at the building at the start in the police car, you can see the tire skidmarks on the road from previous takes. (00:05:05)
Revealing mistake: In the shot where Jack and Harry are descending a ladder to the stranded elevator, we get a good look at the elevator shaft and its surroundings. Upon closer inspection you'll notice how the lowermost section of the shaft is actually a mural - given away by the shadows cast upon it. (00:09:20)
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)
Revealing mistake: When the first bus is blowing up, in the shot of the right side of the bus as it's being engulfed in flames, look closely and you'll see the cable attached to the front of the bus, under its bumper, as it's slowly being pulled by the van in front of it (some DVD or Blu-ray versions may have been edited). (00:26:45)
Revealing mistake: After the first bus explosion when Jack answers the phone, he looks at his watch, which shows 8:05 a.m. If you look closely the watch is on alarm, not regular time. (00:29:05)
Revealing mistake: There is a clear shot of Keanu Reeves' stunt double in the Bronco just before it parks on the highway's shoulder near the gridlocked traffic. (00:34:20)
Revealing mistake: The Jaguar owner holds up the piece of paper with "bomb on bus" written on it, then he loses his grip on it, and when the paper lands directly on the bus windshield, the stunt wires are visible at its sides. (00:37:35)
Revealing mistake: A large crowd of onlookers is visible on the highway overpass in the background as soon as Tuneman props himself up in the driver's seat after crashing into the water barrels. (00:40:10)
Revealing mistake: At the off-ramp's red light Jack stomps on Annie's foot to keep the bus going, but she screams in pain before her foot actually gets crushed. (00:47:15)
Revealing mistake: In the scene after the bus makes the hard right turn, it's moving towards the exit onto the 105 freeway. The bus is surrounded by cop cars and motorcycles - the camera angle is from the air. The bus slams on the brakes to avoid hitting the cop car in front of it - it's only a second but it's obvious the bus wasn't going over 50mph. (00:52:57)
Revealing mistake: If you look at the missing part of the bridge, and look at the shadow underneath, the shadow is of a full bridge. (01:02:35)
Revealing mistake: In the scene before they jump the gap, they played the film in fast forward to make the bus appear to be going faster. If you watch the police on the truck in the background, they are moving in fast forward as well. (01:02:50)
Revealing mistake: When Jack is on the phone with Payne at LAX, a Suburban carrying Lieutenant McMahon and Norwood skids onscreen from between some orange cones. But tire tracks from earlier takes can be seen on the pavement. (01:10:00)
Revealing mistake: In the scene where Jack has come back into the bus from being underneath it, he is talking to Annie about losing gas, if you watch closely he says "Mac" without pressing his button to call him first, then in another shot you see him press it when he starts talking to Mac. (01:15:50)
Revealing mistake: When Ortiz first sees Jack stranded beneath the moving bus, he lifts his head to tell Annie. But the pavement, as seen through the bus floor's opening, is completely motionless. (01:17:25)
Revealing mistake: When Jack and Annie are about to slide out from under the bus, if you look closely when we see Annie hold on to Jack's back, you can see stunt wires visible attached to both characters. (01:29:55)
Revealing mistake: When Jack and Annie are getting off the bus via the access panel on the floor, you can see wheels on the bottom of it. (01:30:00)
Revealing mistake: When Payne opens the money bag in the overhead shot looking down into the bag, as the dye bomb explodes, Payne is wearing a transparent protective face shield, that is partially visible at the top left corner of the screen. Payne is not wearing a face shield in the previous or following consecutive shots. (01:42:35)
Revealing mistake: When Jack is fighting Howard on top of the train, for a second or two, you can see a blue screen behind Howard instead of the regular background. Someone forgot to add the chroma key background in post. (01:44:05)
Answer: Ventilation shafts or roof panels.
Toolio