Actually I think I never used Flash in RBY, because you could see the walls. Next versions though needed that annoying HM. I remember not using Flash because by that time I rarely had 10 Pokémon (I'm not the "Catch'em All" type) and I wasn't in the mood of going through Diglett's Cave to get Flash... xDI did that with Rock Tunnel in Blue, except I actually managed to make it through. It was extremely frustrating and I couldn't believe they'd put that in the game. I felt pretty stupid when my friend told me about Flash.
What about finding a Shiny Milotic in the Battle Pike in Emerald? Arrgghh xDAccidentally killing a Shiny Weelde in HeartGold in the Bug Catching Contest.