I actually once had this issue, and the result was they debited me TWICE – once when I called it to their attention, and the second time, when someone else caught the mistake just before the person I reported it to took care of it. SO I had to go through the process of reporting it AGAIN, to ensure that I got back ONE of the debits.
Next time,just send them a check, with the amount, if you are really concerned. Most often, they will catch the error on their own.