Just found this on newmediamedicine but it something I was told on work experiance once
One quite famous story (urban legend?) I am always told is how a man suffered a heart attack in the street and collapsed. A doctor happened to be passing and ran over to help. A crowd of people formed, as always does when somebody collapses. As the doctor is performing CPR, a man pushes through the crowd and shouts "STAND BACK, I AM A TRAINED FIRST AIDER!" and so the crowd parts.
He barges the doctor aside and starts enthusiastically pumping on this guys chest. The doctor calmly says into his ear "when you get to the part where you call for a doctor, I'm stood right here".