Your main character walks away from somewhere with the wrong something or other. Maybe they get off the train and pick up the wrong jacket. Maybe they take the wrong brief case from the office. Perhaps they pick up the wrong present from the Secret Santa.
What do they find out? What do they lose or gain by their mistake? Do they rectify it?
Think about your main characters
What do they look like?
How do they think?
What is their personality like?
What is their greatest desire?
And their greatest fear?
What is making them absent-minded today?
Think about your setting
Give some attention to the time and place.
How will you show time? Is it important for the reader to know what the time is in the story? Time of day? Part of the year?
What about the time the elapses during the story? What is it? Is it important? How will you show it?
Does the place have an impact on the story? Is it almost another character? Can you show it to the reader by writing with your senses?
The shape of the story
Where will you start your story? After the mistake has been made? Or before?
Is the story about how the protagonist resolves the issue? Or is there a deeper issue that also needs resolving? Or do they need to come to terms with something.
Will this be a piece of flash fiction, a short story, a novella or a novel?