An over swing in golf can cause many issues, with the biggest issue being inconsistency. From my view an over swing is when someone moves laterally off the golf ball or shifts their weight so much to the trail leg that the weight gets on the outside of the back foot.
The collapse of the trail side knee is another common sign of a over swing. Once that occurs it is very difficult to make consistent swings and the ball flight can suffer in many different ways and paths.
When discussing an over swing the balance, temp, correct timing all suffer as a result.
I hear many students tell me that they over swing because their swing is too long and I do not think a long swing is a bad thing. A longer golf swing can often translate into a very successful long career and that is often seen on the Professional Tours. A long swing that stays in balance and does not move the player off the ball or over shift their weight to the outside of the trail foot can be a very good thing.