Go to the scene where the Colonial Marines first encounter "the baddies." This is after Sarge torches the surviving colonist as she dies from a chest-burster birthing itself through her sternum. *ouch*

During the scene where you cut to Ripley, Gorman, and Burke in the APC, watch Ripley's headset. While she's arguing with Gorman about recalling the Marines, the headset is on, off, and then it's on again so that she can rip it off in frustration and anger.

I suppose they shot the scene many times and picked out the best parts, and one of the takes must have been accidentally shot without Sigourney Weaver having her headset on.