Other mistake: When Dallas is in the air vent, his and the alien's location are both being tracked with the movement sensor held by Lambert. Whenever he crawls, his movements match up with the sensor. As he climbs down the ladder, his movement on the sensor goes downwards as it keeps in sync with Dallas himself and also shows that anything sensed on the sensor would be lower down on the sensor's monitor if someone or something was located on another level of the vent. The shot after Dallas says "Am I clear Lambert? I want to get the hell out of here" shows the monitor on the sensor and we can see the alien approaching Dallas on the monitor, with both Dallas and the alien on the same level on the air vent. Dallas then climbs down another level, and it is that the alien was in fact a level below Dallas. The problem with this is, if the alien was approaching Dallas from below, then the monitor should have showed the alien to be a few spaces below where it was on the monitor and not the same level as Dallas.