A very small amount will not hurt, but the more over-full it is, the worse it is. As the level gets higher then normal, the piston rod caps or counterbalances will start hitting it (at least on acceleration and stopping) and beat it into foam. Foam will cavitate in the bearings, and actually will not pump well, sometimes even blocking or restricting the oil take-up screen. Enough air in the system will cause your oil pressure gauge (if you have one) to jump back and forth between 0 and 90 psi.
In short, it will cause serious wear.
Some answers have mentioned problems with oil seals. You would have to have really over-filled and have a huge amount of foam to get to that level. It will also not go out of the breather, since that normally comes out of the valve cover or some other high part of the engine.
This drawing gives an idea of the pink end caps and how close they may be to the oil level. (Note: there are a few engines that are inverted, with the cam on the bottom. These are not as critical.)
SOURCE: www.quora.com