1, Topping it off the leader and turning it into a bush.
2. Not thinning it out properly where the limbs do not have room to seretch and grow out and up.
3. Tearing the limb off the tree instead of making a clean cut.
4. Cutting too close to the tree when pruning to the trunk or not close enough leaving a one inch knob for tree to mend.
5. Not pruning out badly sprouted limbs allowing them to grow and misshape the tree.
That is my two cents worth