Erroneous On Demand bill

This month my normal monthly bill was $40 more than normal. When I called ATT to ask about the increased amount, I was told that there was an on demand charge included in my bill. We have never used the on demand and after discussion with the ATT representative, the additional $40 was removed from my bill. The only activity I have had with ATT in the preceding month was a modem replacement after the old unit failed. My question is: is it possible that the on demand movie (or whatever it was) was somehow registered on the replacement modem?  I thought this might be possible if the new modem was actually a reconditioned unit and had not been fully erased. Is this possible? Or could my old modem have been sent to another customer who watched on demand and not fully assigned to that customer?