Revealing mistake: Michael Douglas almost has a head-on collision with a bus as he chases Sharon Stone down the highway. If you look closely as he slams on his brakes, there are many skid marks in that exact same spot in the road from previous takes. (00:41:05)
Revealing mistake: When Michael Douglas pulls his car in front of Jonny Boz' house after the initial murder we hear him come to a screeching halt. If you look closely, perfect tire skid marks can be seen in the exact same spot on the road in front of his police car before it comes to a stop. (00:04:15)
Revealing mistake: When Jeanne Tripplehorn meets with Michael Douglas it's obvious by the flat reflection of the studio lights that she's wearing prop glasses. (00:13:45)
Answer: There's no definitive answer to this and the ending is deliberately ambiguous and open to interpretation. The audience is left to speculate whether or not Catherine kills Nick, or if she intended to kill him but changes her mind because she loves him, or intends to kill him at a much later time, and so on.
raywest ★