Revealing mistake: When the security guard attempts to open the storage room door where the fire has started, he is set ablaze. Look closely, and he is wearing large protective gloves over his hands, plus a special mask. (00:38:15)
Revealing mistake: On the roof during the helicopter rescue scene, there is a panic and some women run to the helicopter trying to land in strong winds. As it explodes, a woman in a slinky dress falls down. Paul Newman helps her up and her dress is whisked up to reveal "stuntwoman" kneepads on her legs. (01:51:15)
Revealing mistake: When Lorrie jumps out of the window, she is replaced by an obvious stuntman: Fireproof gloves appear on her hands, she is wearing a fireproof mask, and her hair is an obvious wig.
Continuity mistake: When Dan dies there's a wide shot of the building with several floors engulfed in flames. When Lorrie is free-falling just two seconds later, for the scenes play in real time - the flames have disappeared. (01:04:53 - 01:06:10)
Continuity mistake: When the elevator is lowering the people, a wide shot shows the building on fire. Couple shots later the flames are nowhere to be seen.
Continuity mistake: When water is cascading outside the building a man jumps out of a window, producing a hole in the glass. When another man jumps out of the window next to that one, the hole in the glass has a different shape and the hole is several centimeters below where it was before.
Revealing mistake: During the party, doors open and a burning man appears. He is wearing fireproof gloves, his face is covered in some dark mask and some sort of awful wig to hide all the safety procedures. He resembles a werewolf. (00:57:20)
Continuity mistake: The light outside the building as seen from the dining hall is dusk, but from the outside it's pitch dark. This swaps back and forth all the time.
Continuity mistake: The people inside the elevator disappear in the wide angle. (01:57:21 - 01:57:51)
Answer: Where the stairs were blown out they could have crossed over to the adjacent stairwell and back.