The user signs up for a free trial on a website, and in doing so they are required to enter their credit card details. When the trial comes to an end, they automatically start getting billed for the paid service. The user is not given an adequate reminder, nor are they given an easy and rapid way of cancelling the automatic renewal.
Put simply, a Roach Motel makes it very easy for a user to get into a certain situation, but then makes it hard for them to get out of it when they realize it is undesirable. Email newsletter unsubscription is a well known example – whereby it is typically easy to subscribe, but much more effort is needed to unsubscribe.