I had a service that I just stopped paying for once, assuming that they'd cancel my account when the money stopped coming. False - they kept billing me then sold my "debt" to a debt-collection firm who called me at work etc.
Never thought of it again for 4-5 years until I was trying to rent a place in my own name. Yep, the retention specialist 5 years ago met his numbers by NOT cancelling me out. So I had something like an extra 12 months of charges plus modem cost plus a credit hit.
Because I really need this place to live in (3 others were depending on it/me) I ended up coughing up a ton of money. Lessons learned.