Mew being obtainable from the back of a truck in Vermillion city in Red/Blue/Yellow.
You could add in a truck outside of the Team Rocket Hideout in Celadon, and then after you clear Team Rocket out of it, the truck is gone, shows up in Vermillion near the SS Anne, if you snoop around a bit you can find a Team Rocket guy trying to lay low who drops a key to the back of the truck when you defeat him.
Back of the truck has some TMs and an encrypted data disk.
Then, if you use Strength on the truck, it reveals a Bond Villain Lair esque Team Rocket hideout built into caverns that function as a submarine dock.
This lair features Lt. Surge battling through various parts of the cavern system along side you, but only if you have already beaten Giovanni at Silph Co.
There are a few places he pops up, and if you've beaten Giovanni at Silph Co., Surge uses his Voltorbs and Electrodes to self-destruct away certain rocks, exposing otherwise secret areas.
One of these secret areas has a boss Team Rocket Lieutenant or Scientist who drops a decryption key when defeated.
Bring the encrypted disk and the decryption program to Bill, who tells you he can decrypt it, but needs a whole lot of power to supercharge an expirimental computer he happens to be working on just for something like this.
To fulfill that, you have to have a Zapdos in your party that knows Thunder.
Put all that together and Bill can decrypt the drive and wow! It's Mew!
Further, Mewtwo will now only actually appear in Cerulean Cave if you have Mew in your party.