Dark Void puts the foe to sleep. In a Double Battle, Dark Void will hit both foes with an accuracy of 80% for each Pokémon.
Now that is devastating. Putting both foes to sleep in one turn with 80% accuracy is totally OP, and Smeargle can almost always get one off with a Focus Sash.
Due to this glitch, the move Sky Drop was banned in Wi-Fi Random Matches starting November 10, 2010.
This glitch can only occur in Double and Triple Battles. It is activated when Gravity is used while two Pokémon are in the semi-invulnerable state after Sky Drop is used. Gravity will bring the user of Sky Drop down, while the target will be stuck in the semi-invulnerable state, unable to move until it is knocked out.
Basically cripples a Pokemon for the whole game if you can pull it off.
The reason Chatter was banned is because you can record your voice saying inappropriate things. It still doesn't make any sense why Chatot was banned though...