So a cop ended her day's duty and returned to her apartment, entered it in the dark, found a black guy in her apartment, warned him and told him to get out, and when he didn't listen she shot him dead. Then she turned on the light, to discover she mistakenly entered the wrong apartment and her apartment was one floor above in this same location.
Crazy fiction? That's what happened last year. Link in comment.
The sunken feeling when you discover you just made a grave mistake.