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The Azurill gender glitch is a recurring glitch; first existing in Generation III of the main series Pokémon games and later appearing in Generation IV and Generation V.
When a female Azurill evolves into a Marill there is a 1 in 3 chance that it will 'change' into a male Marill. This can be explained by the fact that the gender rate of Marill is different to that of Azurill. Whilst Azurill is 75% female, Marill is 50% male and female. The gender of a Pokémon is determined by its hidden personality value and a certain range of values will determine whether it is male or female. When Azurill evolves however, the possible range of values ensuring that Marill is female are reduced so there is a slight chance of a female Azurill evolving into a male Marill.
-  - A video of the Azurill gender glitch on Pokémon Ruby.
-  - A video of the Azurill gender glitch on Pokémon Diamond and Pearl.