The turn in more depth
The turn has three phases: initiation and heading, turning arc and speed, and setup/heading for the next gate. Before we mention anything about an incorrect turn, we should remind ourselves of the aspects of a correct turn. We will leave discussion of adapting these principles for differing terrain as an exercise for the reader.

The Correct Turn
The correct turn is initiated well above the gate, but the arc begins on the rise line. Since this is moderate terrain, the proper "2/3" heading is used. (Steeper, more technical terrain calls for "gate" while less technical and faster terrain calls for "1/3"). The racer transitions into the turn, pressure switching from right to left in this instance and his weight is generally balanced.
When the turn is initiated, the outside ski should be pressured evenly and smoothly, generating a smooth continuous arc that passes the skis just inside the turning panel. Ideally, the pressure in the skis should rise and fall off quickly and smoothly, but not sharply. The less time spent on edge, the better, of course, but the skis cannot be turned and pressured too suddenly or they will chatter and lose speed.
As the racer crosses the rise line again, the pressure is coming off the outside ski and the racer is perfectly setup for the next gate.

The keys to the proper turn, therefore, are:
1. It starts at the rise line and the panel.
2. It consists of one smooth process: transition, weighting, unweighting.
3. It takes the skier close to the panel.

Turns early - slipping.
In this scenario, the skier begins the turn too early and side-slips while approaching the true initiation point of the turn, instead of maintaining balance and skiing straight. This may be caused by a perceived excess speed, but often by a lack of confidence in the turning power of the skis.
Throughout most of the turn, the arc is correct and the line is generally correct, but the slipping added many tenths to the overall time. "The ski is fastest when it carves." Every coach will tell you, in this situation, "Buddy, you've got your foot on the brake."
* Remember: Pressure equals time!
(the more pressure applied, the more time ticks off the clock)

Turns early - double turn
Another classic mistake racers make is to misjudge the initiation point. This mistake comes from paying too little attention during the inspection or not sticking to the game plan made during that time.
In this image below, we can see easily that the racer started the turn too early. When he realized the arc would take him through the panel, he straightened the arc and crossed the rise line in an unpressured stance, otherwise he'd miss the gate. When the time came to actually make the turn, he is forced to turn much too sharply. To use the coaches' phrase, he is too much "across the hill." This requires excess pressure, which wastes speed and time.

Turns late
The final mistake we'll discuss here is that of turning too late. Starting the turn at the rise line, as this racer did, would mean that the racer would not be setup properly for the next gate or might not even make the gate. To correct for this, the racer turns very hard and overpressures the skis, maybe even side-slips and does a little course maintenance in the growing berm. Enough said. 

by SvetSki