Dragon Ball GT

Synopsis

To prevent Omega Shenron from eating the four-star ball and powering up completely, Goku eats it. Then he and Vegeta attempt to fuse again, but this time Omega Shenron knows what fusion is, and stops them. Goku and Vegeta try to fuse over and over again, but Omega Shenron stops them every time. He's too fast and powerful to avoid long enough to fuse. Then Goku and Vegeta use the after-image technique, which distracts Omega Shenron long enough for them to do the fusion dance. However, the dance doesn't work for some reason. Knowing that it won't work again, Omega Shenron lets them try one more time. This time, Goku reverts to his child self. Because he's been fighting longer than Vegeta, he is out of power, and therefore can't fuse. Suddenly, the four-star ball appears on Goku's forehead. (Source: Wikipedia)