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A Dark pattern is a cleverly designed user interface designed to trick people.
I see a lot of web hosts & domain registrars using dark patterns to deceive customers into buying things they don't really need.
Here's an example from FatCow (an EIG company) they automatically tick these boxes on their checkout page.
There's lots of potential things like making it hard to unsubscribe from something, or automatically ticking boxes to make you buy things you don't need (you can read more about some popular companies doing it here: http://wiki.darkpatterns.org/Home)
What others have you seen?