Theres really no definitive answer. It really depends on the TO and how generous you feel at the time. I've been in a situation where someone played down Garchomp LvX, didn't use Dragon Pulse, attached an energy, then remembered about Dragon Pulse. We called a judge over, and the judge said it was up to me whether or not she could use the Power. If it was obvious that they meant to pick it up, but kind of missed, I would think that the judge would say that its ok for them to pick it up. Plus, do you really want to make yourself seem like the jerk who decided to not let them pick up something when they meant to? It's an ethics/spirit of the game kind of thing in my, and my TOs, opinion. But like I said, it really depends on if you feel like letting them take it back, and what kind of person your TO/judge is.