First happened years ago, in a trade I made with my brother. I gave him my Haunter, it evolved, and I got his Alakazam. Pikachu was in my party at the time, but I didn't trade him.
So I go to trade back his Alakazam, and he goes to trade back my Gengar. Mid-trade, his batteries crap out. Now, we all know the cloning glitch: Alakazam should be gone (which he was) and Gengar should have been on both games.
Well, Gengar was on my game…
My brother, on the other hand, had a clone of my Pikachu. The very Pikachu that follows me around. It acted like any traded pokemon- no nickname change available, didn't follow him. But…I didn't trade the Pikachu. He just butted in of his own free will (nosy little…)
My brother later trained this clone, told me it sucked, and gave it back to me in my Crystal. Which was erased (static electricity and gameboy games don't mix).
But this wasn't the end of it. Years later, I hooked up my Blue to my Yellow (the very same Yellow) and entered a link battle. Once again, Pikachu just had to butt in.
Blue sends out a Missingno.
Yellow sends out a Jolteon.
Yellow's screen has a long delay before Missingno comes out, so I quickly tell Blue to switch to my Omanyte. Blue goes to "Waiting".
Missingno finally pops out of the pokeball on Yellow. But instead of getting my fight, items, etc. I get this message:
"Pikachu is trying to learn (mass of glitch text). But Pikachu already knows 4 moves. Delete an older move to make room for (mass of glitch text)?"
Concerned, I pressed no. After a moment's pause, the very same message popped up, with the very same glitch text. I shut both games off.
Once again, Pikachu was in my party, but not on the battlefield. None of my other pokemon are this nosy. And I still can't recreate this link battle scenario; every time I send out Missingno, Yellow crashes.
Why, Pikachu, WHY? He's a jealous little rat, that's for sure…