Revealing mistake: On the three drivers licenses, the date of birth and address is the same on all of them. Also, their birthdays read 3-11-92, but the movie is set in '73.
Continuity mistake: Throughout the movie, the van of teens passes the Blair Meat Co. and the gas station, which are on the right side of the road. Then at the end of the movie when Erin flags down a trucker, he is heading the same direction except the meat company is now on the left side of the road. The gas station, however, is still on the right side. If they were on the same side of the street before, how could this be? They don't show either driver making any turns.
Answer: Erin must have led the police back to the house and then the Hewitt's were taken away so that way evidence could be collected. They must have thought that Leatherface had escaped and was no longer in the house.