Door reverses in mid-opening or closing.

Remove any obstructions in the hinge/spring area. Most units have an automatic reversing feature in the event the door detects or hits an obstruction.
Many units have an "electric eye" or sensor that may become obstructed. If so, remove the obstruction. At times, these sensors may become misaligned. If the light on the sensor is blinking, it means the two sensors are not aligned and need to be adjusted. Once they are aligned, the lights will stop blinking and the obstruction will be eliminated.