I had LOTS of stuff like this... Let me make a list:
Pokémon Red\Blue - I was trying to catch MissingNo.. So, I did the old man trick and I flew to Cinnabar. While surfing, I got a thing called 'M, that looked like MissingNo. and morphed into a Ponyta in the Pokémon mansion. I also got a lvl. 0 Ditto on the Safari Zone.
Pokémon Yellow - I don't know if this is true, but I found an Zubat at route 1. Probably weird, 'cause I was doing stuff with GameShark codes.
Pokémon Crystal - Got a shiny Celebi. Pretty funny, hum? It was hacked, so it could level up and evolve into a Tyranitar. (I've already got a Celebi). It evolved into a, wait for it... Sunkern!
Pokémon Emerald - Hehehe... The old walk-thru-walls cheat is my best friend. Got a ?????????? when first battling a wild Pokémon. I fainted, surely. Then, on the Pokémon Center, the healing animation lasted FOREVER!!! She said just for a few seconds, didn't she? Oh, by the way, my Misdreavus has a Focus Band, so, when it had 1 HP, I gave it a Pomeg Berry, and it got infinite HP... LOL.
Pokémon Diamond\Pearl - While in a double battle, I got my Arceus and an Egg still alive, I fought with Arceus and DPBox (glitch). Too bad I didn't save my game before...
That's pretty it... Hehehe...