I had this happen to me last Saturday in a game. Instinctively as an umpire I looked at 1st base and there was no runner there because he was almost to 2nd base. SO on the droped third strike I stepped back to give the safe sign. The runner reached first base ok as second was stolen but 1st base is occupied at the time of the pitch. I momentary lapse of concentration there. Bottom line be aware of the situation before a pitch happens and you will be ok. We called the runner out and got it correct. Everyone has to see these things first to get it right the next time. I am a big believer in that.