Yep, that's something form the kit. When you ready the projectile to fling, it creates the bullet at the corner of the screen, then it hides the sprite. But, since you're using the after image behavior, it's drawing the sprite of the projectile even so.
How to fix: in your projectile, set the boolean "Activate" to false. Then, in the Slingshot behavior, in the part "If mouse was released", you'd place a block like the following:
If [the boulder that's attached has behavior After Image
tell [the boulder that's collided] to StartAfterImage for behavior After Image
It should fix your problem.
EDIT: Also, in the After Image behavior, place all the content in the "when drawing" inside this:
If <Activate>
Then it's fixed.