So, here's the glitch. If you battle with thay one guy in the Game Freak building who uses a Clefairy, and then use a move like Trick or Thief to take its Leftovers, you get to benefit from the Leftovers during the battle. However, the Leftovers will poof out of existance once the battle ends. Just FWI.
Hahaha, that's not a glitch. That's something that's existed since BW1.
If you battle any in-game trainer, and Trick/Thief/Cover/whatever is used, the items are restored to their original holders after the battle anyway.
This is how I keep my Lucky Eggs after some Illumise in the Stadium keeps stealing them.