Penalty kicks are funny beasts. By any measure, they should be as close to a "gimme" as there is in soccer. In the fifteen years that I played organized ball, I don't recall taking more then about 6 or 7 PKs. And while I don't want to compare the leagues I played in to the pros, PKs never seemed like a big deal. My dad (who was my first coach) always said that a PK was yours to miss and that no keeper in the world could stop a firm well positioned shot. Don't worry about head games or fakes (hell, he said, be a sport: tell the keeper where you will hit it), just aim and shoot it hard.
That said, PKs seem to have the same psychological effect as free throws in basketball. Too much time to think makes a 90% shot, a 60% shot.
So I am seeing what funny, odd, frightening, experiences other may have had in their playing or watching days.
As a final comment, the only PK I vividly remember is the one I missed - hiiting the right post. So close...