Got a pretty funny story about an old Pearl version I had, way back when I was like 12. I left my Gyarados and Exploud in the Day Care for some reason, then I found out the guy had an egg. Thinking it was something I had forgotten from before, I took it, and it hatched into a Magikarp. Turns out the Magikarp had Explosion. Not only can those two Pokemon breed, but neither can even learn flippin' Explosion! Anyway, the weirdest part that not only did this fish have Explosion but it had 999 in each stat. At level 1. So, I did what any rational person would do- challenge my friend to a battle! Somehow the fish lived on zero HP and proceeded to blow up throughout my mate's team, giving me a win. I then decided- yes! This Pokemon will be my juggernaut and I shall task myself with levelling it up to Level 100!
...That was a mistake. As soon as it hit level 2 the EXP Bar just kept going until it was off the screen. I guess the Exploding Magikarp was the angel of death for my game, to herald that it's time had come to go to... Er... Game heaven. As soon as I turned it back on instead of that opening thing that played I got the cutscene from the part where Cyrus summons the Legendary. Then my game bluescreened and refused to work again...
Turns out that it was a bootleg copy anyway, which had always been a bit glitchy (it always froze whenever you saw Turtwig's back sprite, for example, forcing me to choose Piplup) but I figured this was a common thing that must've happened to some people.
So, I guess the moral of the story is, don't buy games from the local market. Unless you find a way to transfer some seriously glitchy and literally game breaking Pokemon from them first, haha!