Mock Up is an enemy in Super Mario World 2: Yoshi's Island.
It appears to be a balloon with a 1-Up text on it. If Yoshi approaches one, it will inflate. However if he eats it before it inflates, he is granted an extra life otherwise it will explode and reveal that it was actually a Fly Guy who disguised himself as the 1-Up balloon. The explosion will hurt Yoshi and the enemy will laugh mockingly while he flies away.