I actually had to do this today. The waitress approached my table with beef and chicken combo fajitas. I ordered beef only. Before she sat down the sizzling platter I said, “Excuse me, I ordered beef only, no chicken.”
That was all it took – she was back in a couple of minutes with the right order, I didn’t dock the tip (honest mistake that she took care of; why penalize her? nobody’s perfect) and we went about our days.