I noticed this a while back and posted about it on another thread here somewhere.
Be sure to check your ending balance from the day before and the beginning balance on the following day. Check Thurs, Fri and your next statement. I had brought this up to IB a few weeks ago and they pointed out that it "was a long story" but to check the "cash balances from day to day" around the day of the erroneous charge. Sure enough, they had added the overcharge back into my cash balance.
Seems to be a glitch somewhere. I'm sure they are working on it. It only affected partials with me as well. I'm certain any real additional charges/commissions would have been noted.