A word that means beneficial for the individual or few but bad for everyone if everyone is doing it?

I think you mean "the tragedy of the commons". From Wikipedia:

[T]he tragedy of the commons is the depletion of a shared resource by individuals, acting independently and rationally according to each one's self-interest, despite their understanding that depleting the common resource is contrary to the group's long-term best interests.

In game theory, there're numerous games that fit OP's description well including those in Talia Ford's and benshepherd's answers. But, in essence, it's just a multiple-player version of prisoner's dilemma.

Another celebrated(and discredited) example that fits equally well is the paradox of thrift. Suppose that an unfavorable shock hits the economy. An individual will be better off by saving more and spending less unilaterally, provided all others don't adapt their spending behaviour. However, if everyone save more in unison( and they have incentive to do so), everyone will be worse off compared to the utility level that they don't.

As far as I know, in Paul Samuelson's Economics(10ed), paradox of thrift is chosen as a notorious example of fallacy of composition. Thus maybe fallacy of composition is the word you are after.

If you are looking for a single word, I guess it could be emergence, though I'm not absolutely sure.

Added: This is an attempt to show why prisoner's dilemma is significant in the philosophical ground. At sometime in our life, in one form or another, we heard of some maxims with a philosophical flavor, say, "People are better off when making their own choices"(Milton Friedman), "The road to economic meltdown is always and everywhere paved by governments' good intentions"(Austrian School), "That government is best which governs not at all", "People pursuing their self-interest are guided by the invisible hand" These dogmas, or widespread misuses of them, though inherently appealing for us, freemen, are debunked by the simple equilibrium analysis of prisoner's dilemma. The lesson is that it's an oversimplication to believe that individual incentive always coincides with social incentive.

Are you certain it was a one-word adjective? I can't think of any adjectives, but in game theory, there are names of some "games" that the philosopher in question might have used, such as stag hunt.

Also, in economics, such situations are addressed, whether more or less directly, by the notions of high opportunity cost, high marginal cost, diminishing returns etc.

This reminds me of the "January Effect". From Investopedia:

A general increase in stock prices during the month of January. This rally is generally attributed to an increase in buying, which follows the drop in price that typically happens in December when investors, seeking to create tax losses to offset capital gains, prompt a sell-off.

Investors exploited this trend in the mid 1900s, but once the general public heard of the January Effect and tried to exploit it themselves by holding their stocks into the new year, the effect was rendered useless.

I think the concept is the Fallacy of Composition :)