Bad sprite

Bulbapedia also has an article about Bad sprite.

A bad sprite (named after bad clone) is a glitch Pokémon with a sprite that causes memory corruption when it is viewed.

Most commonly in Pokémon Red, Blue, and Yellow the sprite will affect the SRAM and possibly corrupt the Hall of Fame.

Other sprites, including those with a dimension width/height value of 0 can freeze the game, but certain glitch Pokémon including Yellow MissingNo., 4 4 (hex:BF) and 4(h4? (hex:ED) can corrupt the current battle (an effect known as a glitch battle not to be confused with a glitch encounter system) in ways such as corrupting what is one the screen or causing glitch music.