One of the stories is called A Stitch In Time and shows time travel in a single, changeable timeline. The time traveler in the story goes back in time and kills serial killers before they can begin their killing spree. She is driven mad because every time she returns, she remembers the original timeline and also has memories of the "new" (changed) timeline and the events which occured as a result of each change.
She explains: "The time traveler remembers not only the changed timeline, but the original one as well. Every time I use the technique, another universe of possibilities slams into my head."