A Mega-Smoking Gun
More here, here and here.
One POSSIBLE explanation is that the timing of the footage was wrong AND that the reporter was standing in front of a blue-screen projecting slightly old footage of WTC7 as she talks about it having collapsed... but, it seems less and less likely this was the case as more people look into this. Certainly the lighting on the reporter, Jane Stanley looks like natural sunlight, not like she's in a studio. So it is probably not bluescreen. Moreover, the link with Stanley dies suddenly at the end of the clip, perhaps the producers saw the goof or saw that Building 7 was about come down.
Remember, many people were told ahead of time WTC7 was coming down, for instance here and here. This BBC footage seems to be a stark example of this foreknowledge-- where they actually jumped the gun and said it collapsed before it was taken down.
(And thanks to "DEMOCRAT" for pointing me to this footage initially.)