When Scully and Mulder are running away from the building in Dallas right before the bomb goes off, Mulder stops running. When Scully realizes that Mulder has stopped, she stops also, and it seems that Gillian Anderson couldn't get a good purchase on the ground because her feet slip a little bit. She finally catches herself, and the slip is a little hard to see because Mulder is slightly in the way, but if you watch her feet, you can see it.