First one was, I was having a trial for the Scotland Schoolboys under 15 team, and was told that 1 of the scouts wanted me in the team, but was overruled by the other 2 - I would like to think that if I had been picked I might have carved out a career in football - which was my goal as a kid.
P.S. that's soccer for our American friends.
Second one was deciding to quit uni after passing all my 1st year exams in Computer Programming - I still think that is the worst decision I ever made in life (Closely followed by marrying my 1st wife

My mates that finished the course are all earning a fortune now - Doh !!!! could have been me.
How might your life have changed if a couple of decisions had gone down a different path?