????? will render itself and all Pokémon under it in the party screen invisible to the PC, and thus cannot be deposited.
If ????? is encountered in the wild and caught, the battle with ????? will continue, and ????? will flee, causing a glitch dimension crash with a completely-white screen. Additionally, it is of note that while the footprint says "256", the Pokédex number is shown as "000". This is because footprint numbers start at 1, and the numbers in "missing" footprints are hardcoded as part of the footprint's image, making them immune to overflows.