Have you ever had a dream that stayed with you and you weren't sure why? What about a false memory or a moment of recognition that you know didn't actually take place? We've all been mistaken before-- your best friend's boyfriend wasn't really flirting with that other girl. That memory of your cousin stealing from the neighbor was actually a dream. The violent fight between your parents that you witnessed as a kid hadn't really gone down like that-- your imagination made up the part with the blood.
But what if you hadn't been mistaken? What if you actually did see what you thought you saw? What if that dream was real? What if your memory was the accurate recorder of events, and everyone else was lying about it?
Think of a moment like this for your character. What's the situation? How much time passes before they realize this thing actually did happen, and how do they find out?
This one's going to take some thinking and creativity. It may be a way to infuse some much needed life into a preexisting story, or the prompt itself could be the entire piece. Have fun, and if something cool comes out of it, share!