This is one of those prompts where the first line is already done for you. Start with a character (or yourself) and the line "You aren't supposed to be here." Is this something spoken, or is it written by the narrator? What comes next?
Is here "earth," and your character unexpectedly survived something horrific? Did your character get rejected from Yale, but he shows up to the classroom anyway? Is your character not supposed to be in the football player's locker room because she's an eighty-year-old woman with dementia?
This prompt can create tension right off the bat, tension that requires questions, answers, and a backstory. Have fun, and have at it!