I found this article from Reader's Digest very interesting, especially the part about the random nature of medical coding. Something to watch out for!

I have also learned the hard way that asking a doctor whether they are "In-Network" with my insurance company is not enough.  Even if the doctor's office believes they are covered, the insurance company does not always agree, so I now call the insurance company every time we see a new doctor.  And if I call the insurance company 3 times, I will not always get the same answer from each agent.

Further, after a recent ER visit at an "in-network" hospital, I received a bill from the doctor who saw us, who happened to be "out of network".  In the middle of an emergency visit, it didn't occur to me to ask every person who walked in the room which insurance they accept....