The design makes it very easy for you to get into a certain situation, but then makes it hard for you to get out of it (e.g. a subscription).
Ticket sales sites Livenation and Ticketmaster have engaged in a roach motel practices for a number of years. When purchasing some types of tickets, they try to sneak a subscription to a magazine (typically Rolling Stone or Entertainment Weekly) into your basket via a trick question on the checkout page, shown below. Note that the design is "opt out" - the purchase is made for you automatically unless you notice the checkbox and realise you need to tick it to opt out.
The roach motel aspect of this design becomes apparent if you realise too late that you've purchased the subscription. The only way to get a rebate is to download a form, print it out fill it in with a pen, put it in an envelope with a stamp and sent it to them via snail mail.