Not having stops in place during the 2000 election. They could not decide if Gore or Bush was president. That was very painful. Thats learning the hard way.
This post has been done before, look at the archives. But ... biggest move I ever missed - I was trading options fulltime on the Q's and TASR but thought MSO might be a good longer term play when it was at $8. Since I didn't normally trade it I bought 1 measly 10 sp LEAP on it for $50 and sold it 2 days later for $200 when the stock stalled at $10. Stock went to $40. Those $50 Leaps were worth $2950 each at expiration.