The "threefold law" of karma, very popular among
new-agey types, states that any
good or evil you do will be returned to you threefold. Give a
homeless man a dollar and someday you will unexpectedly get three, or something just as nice. Kick the
homeless man and you will suffer a
beating thrice as bad (or something equally displeasing).
The problem with this is the buildup over time of good and bad things happening to people.
For simplicity, let's consider a world consisting of just two people- Adam and Eve, nuclear holocaust survivors, whatever. On a given day, say that each of them does three good things and one bad thing. This means that in the future they can expect nine good things and three bad things to happen to them (or nine good things and one really bad thing, or any combination thereof. Whatever).
In daily life, a large number of the really good or really bad things that happen to us are the result of the actions of another person- a gift from a friend, unexpected help from a stranger, a cutting insult from a relative. If we carry this over to the two-person world example, it becomes clear that as each person performs more and more good or bad actions, the other person is compelled to perform reciprocal actions just to keep the laws of karma working. The example is further complicated if we imagine one person doing mostly good and the other person doing mostly evil. In this case, the good person would have to gain threefold for every act of good, but the only other moral agent around would be the evil person, and evil people are not known for their propensity to return kindness with kindness.
But what about karmic returns that are not the result of the actions of another person? One could suppose that a good person who gives a beggar an apple may benefit by walking beneath a tree and finding three fresh windfall apples, more than making up for that which was given away. But how often in the modern world do we really gain unexpected benefits from the inanimate world? Generally when something unexpected happens in the inanimate world, it concerns something breaking- rarely a benefit. Of course there are exceptions, but I think that an honest examination of daily life will show it to be far less common than windfalls that are either the direct result of human kindness or the indirect result of random human acts. In short, good people slip on the ice just as often as evil people.
So if the majority of good and evil that we are the recipients of is caused by other people, we come back to the problem of buildup. Each person would be obliged to commit more and more acts of good or evil to "pay back" the other person or people who are owed threefold for everything they do. Each person's life would become increasingly full of good and evil happening to them. A person that does an equal amount of good and evil would have to be paid back threefold for each act- soon a very boring person would be the center of a veritable tornado of wonderful and terrible events!
As an aside, there is another objection to the threefold law of karma, best put in a conversation I once overheard:
Skeptic: How do you measure good?
Believer: (horrified at such rationalism) Well, you don't!
Skeptic: Then how do you give it a number?