How an entire game is programmed and how it processes data and your player input, along with how the "Bot" determines what items or pathing to put based on one's performance.
In otherwords, I've spent too much time on Left 4 Dead 2 and know how to manlipilate bots (And the A.I director who manages items and the world paths) for the results I want.
And I'm aware of what I can't change.
I have encyclopedic knowledge of more modern movies, actors, directors, fun facts about their productions. But the mindboggling thing is all the quotes I remember--if you quote a line from a movie I've seem more than once in my life I can probably identify the movie its from. I slip movie quotes into normal conversation every day and almost nobody ever notices.
Pretty useless really.