The Celadon Mansion safe spot is a specific location in Pokémon Red, Blue, Yellow, found at the Celadon Mansion. Its uses were documented by TheZZAZZGlitch.
This location contains a 0x50 sub-tile at the top-left corner of the screen.
This makes it useful for removing the dangerous effects of Super Glitch and - if the Pokémon or items menu (or another relevant menu) is opened here. However, in Yellow, it is not possible to carry over the 0x50 sub-tile into battle by opening the menu at the safe spot.