Sosa did that twice in 2001.
At Dodger Stadium, he caught a fly, and lifted his glove up. The ball popped out and went into the stands for a HR. At Comerica, he would've robbed Macias of a HR, but he hit the ball over the fence with his glove
The best blopper I've seen, is in 2004 where Charles Thomas goes after a ball, knocks it up w/ his glove, then accidently hits it over the fence. A fan then reaches for it right when it goes over the fence so Thomas can't rob the batter