At the beginning of The Hunger Games: Catching Fire when Katniss is hunting with Gale, she tries to shoot some birds. When shooting she suddenly imagines a boy getting hit by the arrow, much to her shock. This hallucination is likely part of her PTSD after the Hunger Games.
But I wonder, was this actually a flashback of a particular tribute she killed during the first movie or was this just some random boy she imagined to have killed now?