Most people are right-handed. Some are left-handed. And a few are ambidextrous, comfortable working with either hand. I am none of the above.
I am comfortable doing certain things right-handed and certain things left-handed. This was a disaster for me in Little League because I throw and catch with the same hand. I'm sure this is why I never became a star Major League baseball player. In woodworking, this was more of an advantage than a disadvantage because I was more able to switch hands than someone who was strictly left- or right-handed.
One of the things I did instinctively with my left hand was to grab my tape measure. This meant that when I pulled the tape out with my right hand, the numbers were always upside down. (Ahh, you can see it coming, right?) Since my tape was always upside down, I was always at risk of reading sixes as nines and vice versa. So most of the time, if I made a cutting error it was invariably a three inch error.
One day, while trimming doors, I cut a piece of casing an inch and a half too short. Without thinking, I went right back to the saw and cut off another inch and a half, thereby setting the world right again.