Numerous:
Bill's Secret Garden
Bill is the child of a Clefairy
Talk to Oak a hundred times and he will give you a lv. 100 CHarizard, Venusaur, and Blastoise.
Dragonite in Mewtwo's Cave that evolves into Yoshi
Missingno. is originally Yoshi (yeah, not enough proof)
Oak's Funeral
The truck that gives you PokeGods and Mews
Infinite Rare Candies from the Machop
The SURFBOARD is available by KO'ing the Surfing Pikachu a hundred times without it getting 1 exp.
Wild Groudon appeared! It's Lv. 1
A breeding center and if you breed a lv. 1 Groudon and a lv. 100 Mewtwo (see where this is going?) you get into Johto and Houen!
There is supposed to be a Mewthree by using MOONSTONE on Mewtwo or by placing a Mewtwo and walking exactly 250,000 steps while it's holding a PokeBall.
Also, to the guy that said people believed Gonbe is Snorlax's pre-evolution, he is right, it is. But I heard that you can get the Munchlax (US name for Gonbe) in Red and Blue by catching 20 Snorlax and they will all have sexy time and give you a Muchlax, lv. -1.
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My friend fell for half of these.