I agree - we know she was injured because of the trap door, but no one knows what really happened. Yes, you make a valid point in saying that she must have been aware of the door since an accident happened before, and I'm sure she's more than aware of it. Because of that, you'd think that something must have gone pretty wrong for her to get injured on it. As capnkidd basically said above, maybe the door opened prematurely or a set piece was out of place or anything, so don't blame the performed.
But, as I said above, no one knows what really happened, so I might be the one that's wrong, who knows. I'm not bitching at you or ridiculing you or anything, just stating my opinion
"If you are going to do something, do it well. And leave something witchy."
-Charlie Manson