Encountering 9 seems to be safe, as its front sprite and cry are consistent. Using it, however, is not, as its back sprite takes data from the PC and therefore many PC boxes can glitch up the game when attempted to be sent out by the player. It can replace the battle music with slow, drawn-out screeches and rumbles, eventually locking up the game in a monotone sound. In addition, it often causes map glitches after the battle, ranging from harmless, remediable character sprites walking around the map at random, to the player's sprite disappearing completely, or simply a game crash.
Encountering 9 has also caused the prevention of the move Fly, and the ability to walk through anything in the game, in some cases.
9's long dex entry can be bypassed by seeing an Abra.
It is, like ?/, unique, as it has infinitely many sprites. It possibly pulls its sprite data from the PC boxes. Despite this, its front sprite is constant.