Continuity mistake: When the 1st chandelier fell, we see Peacock and White are standing near the front door. Scarlet then says "look!" Bringing everyone to look at the dead motorist in the study. Peacock and White are suddenly teleported to the study door behind Scarlet, even though they were quite far away with even a couple steps to climb down.
Revealing mistake: Just before the chandelier falls, Col. Mustard quite visibly flinches, even though, realistically, he would have no way of knowing that it was about to crash behind him.
Answer: As Wadsworth explained to everybody, when he saw the motorist at the door, he didn't throw the key away but put it back in his coat pocket. While everyone's attention was on the motorist, Col. Mustard took the key from Wadsworth pocket and replaced it with a different key.