when you look for Pichu in RBY because you found it in FR/LG
When you find a Magikarp at level 5 and catch it with the master ball
when you try to give Porygon2 a downgrade to be a Porygon
when you look at Metangs and Metagross' Pokedex after you get Beldum. Notice that Metangs are made when two Beldum come together and try to trade with you're friends with another one.