I've had two issues with that in the past that I didn't realize made were considered "round trips."
1. On one sell, I forgot to check "AON" and the stock sold in two increments and they charged me with TWO sells.
2. Another incident, I had SEVERAL thousand shares of a stock I was holding for several weeks and it started pulling back some more so I bought more shares. Well I had a sell order in for a portion of my older shares already set and the stock then went up and those shares previous set for sell sold and I was charged with a round trip due to the couple extras I bought earlier that day.