In the scene where Jack (Ed Harris) is leaving to return to his ship Kay (Goldie Hawn) runs from the apartment pleading to him for understanding. He tears himself away and orders the cabbie to take off. As the cab makes a u-turn and heads down the street you see there is no one in the back seat!

And, oh yes, it's the same cab in the next scene when Kay goes to see Lucky (Kurt Russell) at his trailer!