Revealing mistake: In the restaurant, Deeds pushes the opera singer into a table - you can tell the opera man falling, is in fact a stunt double. (00:33:35)
Revealing mistake: Somebody has already pointed out that the footage of Deeds throwing the cats out of the window is obviously not caught on Pam's camera. In addition, notice that there is no guttering/pole to the left of Mrs. Keeling's flat window - it is on the right. Yet on the TV footage, there is a metal pole to the left of her window. Another point is that Mrs. Keeling's flat is on the corner of a street, yet when we see footage of the cats flying out of the window, her flat isn't at the corner of a street, it's in the centre of a row of flats. (00:48:55 - 00:51:45)
Revealing mistake: When the dog catches the cat, which bounces off the trampoline, it is blatantly obvious the dog in fact has a soft toy cat in his mouth. (00:50:05)
Revealing mistake: When Deeds rescues the last cat from the fire, if you look closely, you can see the fire-proof glove he has on his hand. (00:50:45)
Revealing mistake: When Jan and Pam are fighting, you can tell for most of the stunts Winona Ryder isn't doing them, and it's a double. For instance when she runs into Jan, and gets knocked to the floor, and when she gets thrown into a table. This is confirmed on the commentary too. (01:10:50)
Revealing mistake: Deeds says give the money to them, the United Negro College Fund. When the money is handed over, the title on the desk says NAACP. In the credits, an NAACP official is listed but not one for the UNCF.
Revealing mistake: After Deeds kisses Pam he walks on the sidewalk and jumps on garbage cans and falls. When he gets up you can see the stunt double.
Revealing mistake: While Jan is fighting Pam, she lifts her and the replacement stuntman is visible.
Chosen answer: Helicopter performance is dependent on a number of factors, including air density. As altitude increases, air density decreases and the performance of the helicopter falls as well. At the height of Mt Everest, the air density has fallen to a sufficient degree that most helicopters would be unable to function (few helicopters are capable of functioning above 20,000 feet, nine thousand feet lower than the summit of Everest).
Tailkinker ★