The medical thing is important, more than once I've been told my debt is being sent to collections because the hospital and insurance were bickering over who pays what. My wife and I have adopted a policy of not paying until at least 6 months later, or after those two sort it out, since you can never get your money BACK once sent, but until they settle it out there's no way to know what is owed. There has also been a case where something was on my bill to the hospital that was not a rendered service, and having disputed it endlessly, the hospital would still not relent that my 6 yo son had required a breast pump for his treatment.
I've also heard of, particularly gym memberships, being sent to collections because the company had constructed a labyrinth of obstacles to cancelling membership (e.g. Gold's Gym). So people would simply stop paying, and ultimately be sent to collections for non-payment of a service they didn't use. I suspect this form of collections will be on the rise, as the growing trend of writing mandatory recurring payments into contracts increases. I fully support anyone who cancels such things de facto (as long as they actually stop using the service), it's a horrible practice.