Actually, the door opens well before she is stabbed. Listen for it right after Worth says "Endless human stupidity" and they cut to the shot of outside. One could argue that the characters should have heard the door, but that's another story.
Jade, if you spent more time listening to the dialogue instead of the sound effects, then you would know it is obvious that the trap was set to ignore the noise that the doors make.
For one, the door does make a very distinct noise. I may have noticed it because i watch all my movies at a high volume, but it is there. Also, in the special commentary the director mentions how no one notices the door or the man entering the room and says it is a mistake on his part, so that is a genuine slip-up.
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