Using alder to make a bow, cutting the string grooves in the wrong direction, leaving the bow in front of the wood stove too long, making the handle too narrow, mishandling how to work certain woods that are extremely prone to tear-outs, ( serviceberry, hawthorn, birch.) The list goes on and on, but i've figured out what i like best and what works best for me so those kind of mistakes have stopped. The learning never ends.