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Gen II had a pretty interesting one. You see, there's a giant Snorlax sleeping next to Diglett Tunnel, and to wake it up and open the west Kanto you need to do a small quest where you go to the Power Plant, find out that a thief stole a machine part shutting it down, then find that thief in Cerulean, retrieve the part, deliver it back to the Power Plant, then go to Lavender's Radio Tower to earn one more station for your radio - the Poké Flute. Only then you can wake up the Snorlax and reach the other side of Kanto.
You'd think you could just surf from Fuchsia and reach Pallet from there. Without doing any of that. Then the literal roadblock comes in: for no reason at all, there's a lot of unbreakable boulders blocking you from reaching Fuchsia's beach. And the moment you wake up the Snorlax, the boulders disappear.
