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[FreakMediaLP]

[Rapptz]

The simplest way to do this is by overriding Client.on_error and calling a method on your cog, e.g.

class MyBot(commands.Bot):
    async def on_error(self, event, *args, **kwargs):
        cog = self.get_cog('MyCog')
        if cog is not None:
            await cog.custom_on_error(event, *args, **kwargs)