When I started playing pokemon red I thought that:
Holding down the B button in exactly the right moment would give a pokéball 100% catch rate.
The only button that used battery power was the A button, so I would try to save batteries by using the B button as much as possible.
When the guy in Celadon(?I think it is celadon, it´s the town with Misty´s gym) asked "Are you in a hurry" I would always say "Yes", because I weren´t very good at english (I were 6 or 7, and I am not a native english speaker), so I thought he said "Are you hungry" and that he would give me something if I said yes. (I talked to him like 20 times
)
Leveling Missingno up would turn it into 50 different pokemon.
Putting a lvl 5 magikarp in the daycare center until it´s lvl 50 would turn it into a mew.
I also caught 2 lvl 250 snorlax with the Safari zone glitch, and were dissappointed when they leveled up and became lvl 100s