Exploiting bugs and glitches in core series game.

What are the coolest tricks in acquiring Pokemon, items or moves seemingly legitly in Gen I and II? Fun easter eggs or secrets?

Where do you personally draw the line what is considered cheating or a legit tactic?

Is it true that technically any Pokemon that knows Sketch/Transform/Metronome can learn ANY move in game, by abusing Sketching and Trainer Hosue?

GEN III - VI also welcome.