Lots of other situations where that happens. Go read apple forums. People who live near the Canadian board roam onto Fido all the time and all you can do is call up ATT and complain and they will reserve the charges if it truly a case where you are in a location that the phone should not be roaming onto another carrier.
Btw ATT knows which towers you used. So for example if you used a tower 100 miles outside of there coverage area they are not going to refund the charges.