In R/B, there's a glitch that allows you to enter Glitch Cave. I once entered it, but I don't know how I did it.
All the grass on the route I was changed to cave walls and the non-grass place was changed with cave floor! 0_o
It happened after I won a battle with a person, in Red.
Interesting, the game swaped the first tileset with the second.
I got a lv.100 ;003; before Brock with the Mew Glitch :).