I had a major bike accident in 2006 and was taken to a hospital emergency room. The hospital told me they take my insurance. They really meant they'll bill my insurance.

My insurance company didn't have a negotiated rate with the hospital and the treatment was considered out of network. Instead of paying only the maximum out of pocket cost of $4,000 USD per year, insurance only offered 50% and I got hit with a bill for $50,000. It eventually got worked out months later after months of fighting the hospital, doctors, and insurance, but it sucked big time.