Leech Seed and Toxic stacking
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Using Leech Seed when the opposing Pokémon is already badly poisoned causes Leech Seed's damage to increase every turn as if it was Toxic, because Leech Seed and Toxic both use the same damage algorithm (N * max(1, int(0.0625*MaxHP). N usually doesn't increase if the opponent is under Leech Seed without being badly poisoned but badly poisoning the opponent Pokémon first causes it to do so and make Leech Seed sap progressively more damage every turn.
This was 'fixed' as of Generation II.