Continuity mistake: Watch the first kiss scene between Reeves and Theron. Keanu's shirt drops to the floor, yet you can clearly see it on his shoulder in every other shot. The camera switches angles multiple times, so this is easy to spot.
Other mistake: During the scene when Nelson takes Sara home after rescuing the puppies, she asks him in for coffee and he doesn't accept. After she tells him she will accept a ride the next morning, he agrees and turns off his car. One of his headlights does not turn off.