Big Boo Flag

From the Super Mario Wiki
Its hidden position.

The Big Boo Flag is a special Item hidden by The Big Boo in Super Mario RPG: Legend of the Seven Stars. When visited by the Three Musty Fears, Mario is told by The Big Boo that the flag he hid is "Between 'O' and 'A'." The invisible flag is found by pressing the A button between the "O" and "A" in the word "Goal" at the end of the Mushroom Derby racetrack on Yo'ster Isle.