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>If you beat Mewtwo with dragonair in R/B and use a water stone some stupid amount of times before leaving it will evolve into a yoshi. (Pic almost related)
>You can get to Johto in R/B/Y by surfing in areas behind Pallet Towns borders, or alternatively, there are rare pokemon there.
I also vaguely remember one where you get out of Pallet Town with no pokemon, steal Oaks Parcel and it turns out to be Mew - but Police and Oak will be after you and you'll have to fight Oak with just the Mew and your cartridge would break if you lost or something ridiculous like that. I have a friend who to this day believes there's a trick to the whole thing.