Doing something "on accident"

Posted by ESC on March 08, 2007

Today (March 8, 2007) on an early morning NBC news show in the U.S., the commentator talked about an athlete saying he took one of his wife's drugs "on accident." The opposite being doing something "on purpose." My children used to say, "I did it on accident." I thought it was just a childism. Is "on accident" a real expression, commonly used?