Yes, and it was a devastating one. I traded Piplup for a Mesprit when I was stuck in the elite 4. When Mesprit used confusion on Drapion, it didn't had effect. So I traded Mesprit for a Pelipper (French), Because it can fly and surf at the same time. And then I realized, I MISS MY PIPLUP! T_T And finally, I traded a level 26 Cherrim for a level 100 Piplup (Japanese). I used only Piplup and Starly (About Level 21) to battle against the Elite 4. The reason why I had a Starly with me is just in case Piplup faints and I needed to use a Revive/Max Revive on Piplup. That's my story!
In Pokemon Platinum, I almost ditched my Empoleon! I traded my Empoleon for a different Pokemon, and later on, I traded a different pokemon for Empoleon.