When its Pokédex entry is viewed, its cry is a long, never-ending glitch song, which can lock up the game. Viewing its moves can have the same effect. The only way to bypass this is to have a Magnemite registered as "owned" in the Pokédex.
If is leveled up to level 7, the game will freeze.
This glitch Pokémon article is part of Project GlitchDex, a Bulbapedia project that aims to write comprehensive articles on glitches in the Pokémon games.